Liquid crystal display device and method for producing the same

ABSTRACT

A liquid crystal display device of the present invention includes a pair of substrates and a liquid crystal layer provided between the substrates, wherein liquid crystal molecules in the liquid crystal layer have a negative dielectric anisotropy, and the liquid crystal molecules are aligned in a direction substantially vertical to the substrates when no voltage is being applied and are axis-symmetrically aligned in each of a plurality of pixel regions under application of a voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device and amethod for producing the same. More specifically, the present inventionrelates to a liquid crystal display device having wide viewing anglecharacteristics and a method for producing the same.

2. Description of the Related Art

In the past, a liquid crystal display device (hereinafter, also referredto as an “LCD”) in a twisted nematic (TN) mode has been known. Theliquid crystal display device in a TN mode has poor viewing anglecharacteristics (i.e., a narrow viewing angle). As shown in FIG. 30A,when TN-LCD 200 is in a gray-scale display, liquid crystal molecules 202are tilted in one direction. As a result, in the case where TN-LCD 200is observed in viewing angle directions A and B as shown in FIG. 30A,apparent light transmittance varies depending upon the direction.Accordingly, the display quality (e.g., contrast ratio) of TN-LCD 200greatly depends upon the viewing angle.

In order to improve the viewing angle characteristics of a liquidcrystal display device by controlling the alignment state of liquidcrystal molecules, it is required to align liquid crystal molecules inat least two directions in each pixel. Examples of such liquid crystaldisplay devices includes those in an axially symmetric aligned microcell(ASM) mode in which liquid crystal molecules are axis-symmetricallyaligned in each pixel. Referring to FIG. 30B, for example, when a liquidcrystal display device 210 in an ASM mode in which a liquid crystalregion 214 is surrounded by a polymer region 212 is in gray scales,liquid crystal molecules are aligned in two different directions. In thecase where the liquid crystal display device 210 is observed in viewingangle directions represented by arrows A and B, apparent lighttransmittance is averaged. As a result, the light transmittance in theviewing angle directions A and B becomes substantially equal, wherebyviewing angle characteristics are improved compared with those in a TNmode.

Examples of liquid crystal display devices in a mode having improvedviewing angle characteristics (hereinafter, referred to as a “wideviewing angle mode”) including an ASM mode will be described below.

(1) There is a technique for electrically controlling a transparentstate or an opaque state by utilizing birefringence of a liquid crystalmaterial in a liquid crystal display device which has polymer walls in aliquid crystal cell without polarizing plates and which does not requireany alignment treatment. According to this technique, the ordinary indexof liquid crystal molecules is matched with the refractive index of asupporting medium. Under the application of a voltage, the liquidcrystal molecules are aligned, whereby a transparent state is displayed.When no voltage is being applied, the alignment of the liquid crystalmolecules is disturbed, whereby a light scattering state is displayed.

For example, Japanese National Phase PCT Laid-open Publication No.61-502128 discloses a technique for mixing liquid crystal with aphotocurable or thermosetting resin, curing the resin to phase-separateliquid crystal from the resin, thereby forming liquid crystal dropletsin the resin. Furthermore, Japanese Laid-open Publication Nos. 4-338923and 4-212928 disclose a liquid crystal display device in a wide viewingangle mode obtained by combining the device disclosed in JapaneseNational Phase PCT Laid-open Publication No. 61-502128 with polarizingplates in such a manner that polarization axes are orthogonal to eachother.

(2) As a technique for improving viewing angle characteristics of anon-scattering type liquid crystal cell using polarizing plates,Japanese Laid-open Publication No. 5-27242 discloses a technique forproducing a composite material containing liquid crystal and a polymermaterial from a mixture of liquid crystal and a photocurable resin byphase separation. According to this technique, the liquid crystalmolecules in liquid crystal domains are randomly aligned by generatedpolymers, the liquid crystal molecules rise in different directions ineach domain under the application of a voltage. Therefore, the apparentlight transmittance observed in each direction becomes substantiallyequal (because retardation d·Δn is averaged, where d is a thickness of aliquid crystal layer and Δn is birefringence of a liquid crystalmaterial), so that the viewing angle characteristics in gray scales areimproved.

(3) Recently, in Japanese Laid-open Publication No. 7-120728, theinventors of the present invention have proposed a liquid crystaldisplay device in which liquid crystal molecules are omnidirectionallyaligned (e.g., in a spiral state) in pixel regions by controlling lightusing a photomask or the like during photopolymerization. This deviceuses a technique of axis-symmetrically aligning liquid crystal moleculesby utilizing phase separation from a mixture of liquid crystal and aphotocurable resin. The liquid crystal molecules are axis-symmetricallyaligned when no voltage is being applied, and come closer to homeotropicalignment (alignment vertical to the substrates) under the applicationof a voltage, whereby the viewing angle characteristics are remarkablyimproved. This technique is a p-type display mode using a p-type liquidcrystal material (i.e., a material with a positive dielectric anisotropyΔε).

As an example of a method for producing a device as described above,Japanese Laid-open Publication No. 8-95012 discloses a method forforming lattice-shaped polymer walls having a thickness smaller than thecell thickness in each pixel region, injecting a mixture of liquidcrystal and a photocurable resin into the cell thus produced, andaxis-symmetrically aligning liquid crystal molecules by utilizingtwo-phase regions in which a liquid crystal phase and a uniform phaseexist. This production method does not use alignment films.

(4) Furthermore, Japanese Laid-open Publication No. 6-308496 discloses aliquid crystal display device in a wide viewing angle mode including anaxis-symmetrical alignment film made of a crystalline polymer with aspherulite structure on the surface of a substrate.

(5) Japanese Laid-open Publication No. 6-194655 discloses a techniquefor coating an alignment film on a substrate and aligning liquid crystalmolecules in a random direction without performing an alignmenttreatment such as rubbing.

There are techniques for dividing pixels into a plurality of regions andaligning liquid crystal molecules in each region in such a manner thatthe viewing angle characteristics in each region compensate for eachother. Examples of the method will be described below.

(6) Japanese Laid-open Publication No. 63-106624 discloses a method fordividing each pixel into regions and performing an alignment treatmentsuch as rubbing so that the rubbing directions in the respective regionsbecome different.

FIGS. 31 and 32 show a liquid crystal display device obtained by theabove method, having wide viewing angle characteristics and beingcapable of obtaining a display with a satisfactory contrast. FIG. 31 isa schematic plan view of the liquid crystal display device, and FIG. 32is a cross-sectional view taken along the E-E′ line in FIG. 31.

A pixel electrode (transparent electrode) 520 provided on each pixel, analignment film 510, and a thin film transistor 513 driving the pixelelectrode 520 are provided on one glass substrate 522 of the liquidcrystal display device. A counter electrode (transparent electrode) 519and an alignment film 509 are provided on the other glass substrate 521.The alignment films 509 and 510 are made of polyimide. A pixel B definedby the opposing transparent electrodes 519 and 520 is a square of 200μm, for example, and a plurality of pixels B are arranged in a matrix. Aband-shaped spacer 523 made of polyimide is provided in a center portionof the pixel electrodes 520, as a result of which each pixel B isdivided into regions I and II by the band-shaped spacer 523.

The regions I and II are formed as schematically shown in FIG. 33. Theglass substrates 521 and 522 are respectively subjected to a rubbingtreatment in the arrow directions as shown in FIG. 33. In the past, inthe case of providing the regions I with an alignment regulating force,the substrate 521 is subjected to a rubbing treatment with the regionsII covered with a resist. Similarly, in the case of providing theregions II with an alignment regulating force, the substrate 521 issubjected to a rubbing treatment with the regions I covered with aresist.

According to the above technique, the alignment directions of liquidcrystal molecules in the respective regions have the same spiral-typetwist direction but form different angles with respect to the surface ofthe substrates. Due to the difference in angle with respect to thesurface of the substrates, the liquid crystal molecules rise indifferent directions under the application of a voltage. Therefore, inthe case where light is incident upon the substrate in a directiontilted from a direction normal to the substrate, the opticalcharacteristics of the respective regions compensate for each other. Asa result, the viewing angle dependence under the application of avoltage is cancelled in the regions having different orientations ineach pixel between the substrates. Thus, optical characteristics withless viewing angle dependence are obtained. In particular, even when aviewing angle is varied in gray scales, there will be no phenomenon ofgray-scale inversion.

(7) As a technique for making an alignment direction of an alignmentfilm different, Japanese Laid-open Publication Nos. 7-199193 and7-333612 disclose a technique for forming unevenness having a tilt ineach pixel, thereby making the direction in which liquid crystalmolecules are tilted different depending upon the region in each pixel.According to this technique, a pretilt angle is varied on a regionalbasis due to the different tilt directions in each pixel, thereby makingthe direction in which the liquid crystal molecules are tilteddifferent. Thus, the viewing angle characteristics of a liquid crystaldisplay device are improved. Japanese Laid-open Publication No. 7-199193also discloses a homeotropic liquid crystal display device which uses ann-type (Δε<0) liquid crystal material and a homeotropic alignment film,and in which liquid crystal molecules are aligned in a directionvertical to substrates when no voltage is being applied and tilted in adirection horizontal to the substrates under the application of avoltage.

(8) Furthermore, Japanese Laid-open publication No. 6-301036 hasproposed a liquid crystal display device having wide viewing anglecharacteristics and being capable of obtaining satisfactory displayquality. FIG. 34 is a perspective view showing an external appearance ofthe liquid crystal display device, and FIG. 35 is a schematiccross-sectional view thereof. The liquid crystal display device includesa liquid crystal layer 612 having vertically aligned liquid crystalmolecules 612A between a pair of electrode substrates. Pixel electrodes611 are provided on one substrate 610, and counter electrodes 613 areprovided on the other substrate (not shown). Each counter electrode 613has openings 614 corresponding to central portions of each pixel.

The liquid crystal molecules 612A in a region of a liquid crystal layercorresponding to the opening 614 are stable, being vertically alignedunder the application of a driving voltage. The liquid crystal molecules612A on the periphery of the region corresponding to the opening 614 arealso stable in alignment due to the interaction with the liquid crystalmolecules 612A in the region corresponding to the opening 614. As aresult, the liquid crystal molecules 612A in each pixel are aligned soas to face the central portion of the pixel corresponding to the opening614. Thus, if the opening 614 of each pixel is provided at the identicalposition (e.g., a central portion of each pixel), the liquid crystalmolecules are aligned similarly in each pixel. Because of this, even ifa disclination line is similarly generated in each pixel, roughness of adisplay can be prevented. In FIG. 35, the reference numerals 615 and 616denote gate bus lines, and 617 and 618 denote homeotropic alignmentfilms.

Liquid crystal display devices (e.g., TFT-LCD) have been widely used asflat displays. However, large TFT-LCDs of a 20-inch or more diagonalscreen, whose application for wall mounting has been expected, have notbeen commercially available. In recent years, as a candidate forrealizing a large display device, a plasma address LCD (PALC) disclosedin Japanese Laid-open Publication No. 1-217396 has received attention.

FIG. 36 shows a cross-sectional structure of a PALC. A PALC 700 includesa liquid crystal layer 702 between a pair of substrates 701 and 711. Aplurality of plasma chambers 713 are disposed between the substrate 711and the liquid crystal layer 702. Each plasma chamber 713 is defined bythe substrate 711, a dielectric sheet 716 opposing the substrate 711,and partition walls 712 provided between the substrate 711 and thedielectric sheet 716. Gas (e.g., helium, neon, etc.) sealed in theplasma chamber 713 is ionized by applying a voltage across an anode 714and a cathode 715 formed on the surface of the substrate 711 in theplasma chamber 713, whereby plasma discharge occurs.

A plurality of plasma chambers 713 extend in the shape of stripes in adirection vertical to the drawing surface of FIG. 36 in such a manner asto be orthogonal to transparent electrodes 705 provided on the surfaceof the substrate 701 on the liquid crystal layer 702 side. Compared witha simple matrix type liquid crystal display device, the transparentelectrodes 705 correspond to display electrodes (signal electrodes) andthe plasma chambers 713 correspond to scanning electrodes. The substrate711, the dielectric sheet 716, the plasma chambers 713, etc. arecollectively called a plasma substrate 710.

Referring to FIG. 37, the basic principle of the PALC 700 will bedescribed. The plasma chambers 713 are successively turned on, and thegas in the selected plasma chamber 713 is ionized. As shown in FIG. 37,under the condition that the plasma chamber 713 is ionized, a charge, inaccordance with a voltage supplied from the signal lines to thetransparent electrodes 705, is accumulated and held on a reverse surfaceof the dielectric sheet 716 on the plasma chamber 713 side. Thus, asignal voltage supplied from the signal lines is applied to a region ofthe liquid crystal layer 702 positioned above the ionized plasma chamber713. When the plasma chamber 713 is not ionized, the charge is notsupplied to the reverse surface of the dielectric sheet 716. Therefore,the signal voltage is not supplied to the region of the liquid crystallayer 702 positioned above the plasma chamber 713. Thus, the plasmachambers 713 function as scanning electrodes in a simple matrix typeliquid crystal display device.

As a technique for producing a display with a large screen, JapaneseLaid-open Publication No. 4-265931 discloses a technique of forming aplasma chamber structure on a glass substrate by a printing method usingglass paste.

Japanese Laid-open Publication No. 4-313788 discloses a structure inwhich transparent electrodes are patterned in a direction of plasmachambers. In this structure, even when a thick dielectric sheet isinterposed between plasma chambers and a liquid crystal layer for thepurpose of enhancing the strength of the dielectric sheet, charge isprevented from dispersing on the liquid crystal layer side to causebleeding of a display.

The above-described techniques have respective problems. Hereinafter,these problems will be described.

In the conventional liquid crystal display device in an ASM mode, aliquid crystal material with a positive dielectric anisotropy Δε isused. In this display mode, as described above, liquid crystal moleculesare axis-symmetrically aligned, so that outstanding displaycharacteristics are obtained in an omnidirection. However, this liquidcrystal display device has the following problems (1) to (4): (1) sincethis display mode is a normally white (NW) mode, a relatively highdriving voltage is required for decreasing the light transmittance underthe application of a voltage so as to obtain a high contrast; (2) inorder to prevent light leakage when no voltage is being applied, it isrequired to prescribe an area of each light-blocking portion (e.g., ablack matrix (BM)) to be large; (3) the liquid crystal display device inan ASM mode is difficult to produce, because a phase separation steprequiring complicated temperature control is used for forming an ASMmode; and (4) since the liquid crystal display device in an ASM mode isdifficult to produce, it is difficult to control the position of eachcentral axis around which liquid crystal molecules are symmetricallyaligned, the position of the central axis is varied depending upon thepixel, and the central axis is not positioned almost at the center ofthe pixel region; as a result, when the liquid crystal display device isobserved in an oblique direction, a rough display with unsatisfactoryquality is obtained.

Furthermore, in the liquid crystal display devices using a liquidcrystal material with a positive dielectric anisotropy Δε as describedin the above-mentioned (6) and (7), alignment directions of the liquidcrystal molecules on the division lines become discontinuous under theapplication of a voltage, i.e., disclination lines are generated,causing the decrease in contrast ratio. Furthermore, in this liquidcrystal display device, in order to produce a plurality of dividedregions, a resist is coated onto an alignment film, followed by rubbingon a region basis. According to this method, the alignment film isexposed to a resist material, a developing solution, a release agent,etc. Therefore, ions contained in the resist, the developing solution,the release agent, etc., remain on the alignment film after the resistis peeled off. The remaining ions may have an adverse effect on thedisplay characteristics by moving during the operation of the liquidcrystal display device to deteriorate the charge-holding characteristicsof the liquid crystal material and to cause a phenomenon such as animage burn-in. Furthermore, depending upon the kinds of the alignmentfilm and the resist to be combined, the alignment film is damaged tolose an alignment regulating force. Thus, such a liquid crystal displaydevice is low both in production efficiency and production stability.

Furthermore, in the liquid crystal display device described in the above(8), the liquid crystal molecules are axis-symmetrically aligned only inthe opening of the counter electrode. More specifically, the liquidcrystal molecules on the periphery of the pixel away from the openingare not axis-symmetrically aligned. Thus, the liquid crystal moleculesare randomly aligned, which may cause a rough display. Furthermore, thepositions or sizes of liquid crystal domains (regions where thealignment direction of the liquid crystal molecules are continuous, anddisclination lines are not generated) are not defined, so thatdisclination lines cannot be prevented from being generated in pixels,particularly, causing a rough display in gray scales.

The PALC has the following problems. The PALC mainly uses a TN mode.When a TN mode in which display quality depends upon a viewing angle isapplied to a display device with a large screen, even when an observer'sposition is fixed, the viewing angle (a and b) is varied depending uponthe position of a display screen to be observed, as shown in FIG. 38.Therefore, the display quality becomes unsatisfactory in the displayscreen.

In the case of the PALC in a TN mode, considering the viewing angledependence of the TN mode, polarization axes of polarizing plates areset at 45° from a crosswise direction on the display surface, therebyadjusting the sideward viewing angle characteristics seen by an observerin a satisfactory direction. In this case, at a portion such as anattachment surface between the plasma substrate and the thin glass sheetwhere the difference in refractive index is present, an attachmentportion becomes visible due to the birefringence and the difference inrefractive index of polarized light on the attachment surface, wherebylight leakage, which is critical to a display, occurs in a crosswisedirection.

The PALC uses a display mode using p-type liquid crystal material, suchas a NW mode and a TN mode. In the PALCs in these display modes, asufficient contrast ratio cannot be obtained. This is caused by thenonuniform voltage (electric field) applied to the liquid crystal layerdue to the nonuniform plasma charge. In the NW mode using p-type liquidcrystal (Δε>0), particularly, a black level under the application of avoltage is decreased, resulting in a great decrease in contrast ratio.

SUMMARY OF THE INVENTION

A liquid crystal display device of the present invention includes a pairof substrates and a liquid crystal layer provided between thesubstrates, wherein liquid crystal molecules in the liquid crystal layerhave a negative dielectric anisotropy, and the liquid crystal moleculesare aligned in a direction substantially vertical to the substrates whenno voltage is being applied and axis-symmetrically aligned in each of aplurality of pixel regions under application of a voltage.

In one embodiment of the present invention, a thickness (d_(in)) of theliquid crystal layer in the pixel region is larger than a thickness(d_(out)) of the liquid crystal layer outside of the pixel region, andthe device includes a homeotropic alignment layer in a regioncorresponding to the pixel region on a surface of at least one of thesubstrates on the liquid crystal layer side.

In another embodiment of the present invention, at least one of thesubstrates has convex portions defining the pixel region on a surface onthe liquid crystal layer side.

In another embodiment of the present invention, the thickness of theliquid crystal layer in the pixel region is largest at a central portionof the pixel region and continuously decreases toward a peripheralportion of the pixel region.

In another embodiment of the present invention, the thickness of theliquid crystal layer in the pixel region is axis-symmetrically changedaround the central portion of the pixel region.

In another embodiment of the present invention, the above-mentionedliquid crystal display device further includes a projection at thecentral portion of the pixel region, wherein the liquid crystalmolecules are axis-symmetrically aligned around the projection under theapplication of a voltage.

In another embodiment of the present invention, a retardation d·Δn ofthe liquid crystal layer is in a range of about 300 nm to about 500 nm.

In another embodiment of the present invention, a twist angle of theliquid crystal layer is in a range of about 45° to about 110°.

In another embodiment of the present invention, the above-mentionedliquid crystal display device includes a pair of polarizing platesdisposed in crossed Nicols on both sides of the liquid crystal layer,and a phase difference plate having a relationship, in which arefractive index n_(x,y) in an in-plane direction is greater than arefractive index n_(z) in a direction vertical to a plane, is providedon at least one of the polarizing plates.

In another embodiment of the present invention, an axis-symmetricalalignment fixing layer which provides the liquid crystal molecules withan axis-symmetrical pretilt angle is further formed on a surface of atleast one of the substrates on the liquid crystal layer side.

In another embodiment of the present invention, the axis-symmetricalalignment fixing layer contains a photocurable resin.

A method for producing a liquid crystal display device of the presentinvention includes the steps of: forming a homeotropic alignment layeron a pair of substrates, respectively; disposing a mixture containing aliquid crystal material having a negative dielectric anisotropy and aphotocurable resin between the homeotropic alignment layers on thesubstrates; and curing the photocurable resin while applying a voltagehigher than a threshold voltage of the liquid crystal material to themixture, so as to form an axis-symmetrical alignment fixing layerproviding the liquid crystal molecules with an axis-symmetrical pretiltangle.

In one embodiment of the present invention, the above-mentioned methodfurther includes the step of forming convex portions defining pixelregions on a surface of at least one of the substrates before the stepof forming the homeotropic alignment layers on the substrates.

A liquid crystal display device of the present invention includes: aplasma substrate having plasma chambers for performing plasma discharge;a counter substrate having signal electrodes; and a liquid crystal layerprovided between the plasma substrate and the counter substrate, thedevice being driven by the signal electrodes and the plasma chambers,wherein liquid crystal molecules in the liquid crystal layer have anegative dielectric anisotropy, and the liquid crystal molecules arealigned in a direction substantially vertical to the substrates when novoltage is being applied and axis-symmetrically aligned in each of aplurality of pixel regions under application of a voltage.

In one embodiment of the present invention, a thickness (d_(in)) of theliquid crystal layer in the pixel region is larger than a thickness(d_(out)) of the liquid crystal layer outside of the pixel region, andthe device includes a homeotropic alignment layer in a regioncorresponding to the pixel region on a surface of at least one of thesubstrates on the liquid crystal layer side.

In another embodiment of the present invention, at least one of thecounter substrate and the plasma substrate has convex portions definingthe pixel region on a surface on the liquid crystal layer side.

In another embodiment of the present invention, the thickness of theliquid crystal layer in the pixel region is largest at a central portionof the pixel region and continuously decreases toward a peripheralportion of the pixel region.

In another embodiment of the present invention, the thickness of theliquid crystal layer in the pixel region is axis-symmetrically changedaround the central portion of the pixel region.

In another embodiment of the present invention, the above-mentionedliquid crystal display device includes a pair of polarizing platesdisposed in crossed-Nicols on both sides of the liquid crystal layer, apolarization axis of one of the polarizing plates being parallel to anextending direction of the signal electrodes or the plasma chambers.

In another embodiment of the present invention, an axis-symmetricalalignment fixing layer which provides the liquid crystal molecules withan axis-symmetrical pretilt angle is further formed on a surface of atleast one of the plasma substrate and the counter substrate on theliquid crystal layer side.

In another embodiment of the present invention, the axis-symmetricalalignment fixing layer contains a photocurable resin.

A liquid crystal display device of the present invention includes: apair of substrates and a liquid crystal layer provided between thesubstrates, wherein liquid crystal molecules in the liquid crystal layerhave a negative dielectric anisotropy, and the liquid crystal moleculesare aligned in a direction substantially vertical to the substrates whenno driving voltage is being applied and axis-symmetrically alignedaround an axis-symmetrical alignment central axis in each of a pluralityof pixel regions under application of a driving voltage, and convexportions defining the pixel region are provided on a surface of at leastone of the substrates on the liquid crystal layer side, and a treatmentfor controlling a position of the axis-symmetrical alignment centralaxis is conducted.

In one embodiment of the present invention, the above-mentioned liquidcrystal display device includes a region in which the liquid crystalmolecules keep a homeotropic alignment state under application of anaxis-symmetrical alignment central axis forming voltage at eachpredetermined position in the plurality of pixel regions.

In another embodiment of the present invention, Sa is an area of theregion in which the liquid crystal molecules keep a homeotropicalignment state under the application of the axis-symmetrical alignmentcentral axis forming voltage, A is an area of the pixel region, and Sa/Asatisfies the relationship 0<Sa/A<4%.

In another embodiment of the present invention, the above-mentionedliquid crystal display device includes an axis-symmetrical alignmentcentral axis forming portion at a predetermined position in each of theplurality of pixel regions, and the axis-symmetrical alignment centralaxis of the liquid crystal molecules is formed corresponding to theaxis-symmetrical alignment central axis forming portion.

In another embodiment of the present invention, Sb is an area of theaxis-symmetrical alignment central axis forming portion, A is an area ofthe pixel region, and Sb/A satisfies the relationship 0<Sb/A<4%.

In another embodiment of the present invention, a thickness of theliquid crystal layer in the pixel region is larger than a thickness ofthe liquid crystal layer outside of the pixel region.

In another embodiment of the present invention, the thickness of theliquid crystal layer in the pixel region is largest at a central portionof the pixel region and continuously decreases from the central portionto a peripheral portion of the pixel region.

In another embodiment of the present invention, the thickness of theliquid crystal layer in the pixel region is axis-symmetrically changedaround the central portion of the pixel region.

In another embodiment of the present invention, an axis-symmetricalalignment fixing layer is provided on a surface of at least one of thesubstrates on the liquid crystal layer side.

In another embodiment of the present invention, the axis-symmetricalalignment fixing layer contains a photocurable resin.

A method for producing a liquid crystal display device is provided. Thedevice includes a pair of substrates and a liquid crystal layer providedbetween the substrates, liquid crystal molecules in the liquid crystallayer having a negative dielectric anisotropy, the liquid crystalmolecules being aligned in a direction substantially vertical to thesubstrates when no driving voltage is being applied and beingaxis-symmetrically aligned around an axis-symmetrical alignment centralaxis in each of a plurality of pixel regions under application of adriving voltage. The method includes the step of performing anaxis-symmetrical alignment central axis forming process.

In one embodiment of the present invention, the axis-symmetricalalignment central axis forming process includes the steps of: disposinga precursor mixture containing a liquid crystal material and aphotocurable material between the substrates; and curing thephotocurable material while applying an axis-symmetrical alignmentcentral axis forming voltage to the precursor mixture.

In another embodiment of the present invention, the axis-symmetricalalignment central axis forming voltage is ½ or more of a thresholdvoltage of the liquid crystal material.

In another embodiment of the present invention, the axis-symmetricalalignment central axis forming voltage is an AC voltage.

In another embodiment of the present invention, a frequency of the ACvoltage is 1 Hz or more.

Thus, the invention described herein makes possible the advantages of(1) providing a liquid crystal display device including a liquid crystalregion in which liquid crystal molecules are axis-symmetrically alignedin each pixel region, having outstanding viewing angle characteristicsin an omnidirection and a high contrast without roughness; (2) providinga plasma address LCD having outstanding viewing angle characteristicsand a high contrast; and (3) providing a method for producing the liquidcrystal display devices as described above with ease.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1D are schematic views illustrating the operationprinciple of a liquid crystal display device in an embodiment of thepresent invention.

FIG. 2 is a graph showing a voltage-transmittance curve of the liquidcrystal display device shown in FIGS. 1A through 1D.

FIGS. 3A through 3D are schematic views illustrating the relationshipbetween the position of a central axis of an axis-symmetrically alignedregion and the display quality.

FIGS. 4A and 4B are schematic views illustrating a thickness d_(in)(x)of a liquid crystal layer of the liquid crystal display device in aembodiment of the present invention.

FIGS. 5A through 5C are schematic cross-sectional views illustrating apixel region in the liquid crystal display device in the embodiment ofthe present invention.

FIG. 6 is a graph showing a voltage-transmittance curve of a liquidcrystal display device including a liquid crystal layer with d·Δn=450nm.

FIG. 7 is a schematic cross-sectional view illustrating an embodiment ofa PALC of the present invention.

FIG. 8A is a radar chart showing viewing angle characteristics of aliquid crystal display device in a TN mode, and

FIG. 8B is a schematic view illustrating the arrangement of polarizingplates in the liquid crystal display device in a TN mode.

FIG. 9 is a radar chart showing viewing angle characteristics of a PALCof the present invention.

FIGS. 10A through 10D are schematic views illustrating the basicstructure and operation principle of a liquid crystal display device inanother embodiment of the present invention.

FIG. 11A is a schematic view showing a state of an electric fielddistribution when a voltage is applied to the liquid crystal displaydevice in the embodiment of the present invention, and

FIG. 11B is a schematic view showing an alignment state of liquidcrystal molecules when a voltage is applied to the liquid crystaldisplay device shown in FIG. 11A.

FIG. 12A is a schematic partial cross-sectional view of a substrate usedin a liquid crystal display device in Example 1 of the presentinvention, and

FIG. 12B is a plan view thereof.

FIG. 13 is a graph showing electro-optic characteristics of the liquidcrystal display device in Example 1 of the present invention.

FIG. 14 is a radar chart showing viewing angle characteristics of theliquid crystal display device in Example 1 of the present invention.

FIG. 15 is a schematic partial cross-sectional view of a substrate usedin a liquid crystal display device in Example 2 of the presentinvention.

FIG. 16 is a radar chart showing viewing angle characteristics of aliquid crystal display device in Example 7 of the present invention.

FIG. 17 is a schematic partial cross-sectional view of a liquid crystaldisplay device in Example 8 of the present invention.

FIG. 18 is a schematic partial cross-sectional view of a substrate usedin the liquid crystal display device in Example 8 of the presentinvention.

FIG. 19A is a schematic partial cross-sectional view of a substrate usedin a PALC in Example 11 of the present invention, and

FIG. 19B is a plan view thereof.

FIG. 20 is a schematic partial cross-sectional view of a PALC in Example12 of the present invention.

FIG. 21 is a schematic partial cross-sectional view of a substrate usedin the PALC in Example 12 of the present invention.

FIG. 22A is a schematic partial cross-sectional view of a liquid crystaldisplay device in Example 13 of the present invention, and

FIG. 22B is a plan view of one pixel therein.

FIG. 23 is a schematic view showing results obtained by observing pixelsof a liquid crystal cell produced in Example 13 of the present inventionwith a polarizing microscope in crossed-Nicols.

FIG. 24 is a radar chart showing viewing angle characteristics of theliquid crystal display device in Example 13 of the present invention.

FIG. 25A is a schematic partial cross-sectional view of a liquid crystaldisplay device in Example 14 of the present invention, and

FIG. 25B is a plan view of one pixel therein.

FIG. 26 is a schematic partial cross-sectional view of a liquid crystaldisplay device in Example 16 of the present invention.

FIG. 27 is a radar chart showing viewing angle characteristics of aliquid crystal display device in Example 18 of the present invention.

FIG. 28 is a radar chart showing viewing angle characteristics of aliquid crystal display device in Example 19 of the present invention.

FIG. 29 is a schematic partial cross-sectional view of a liquid crystaldisplay device in Comparative Example 10.

FIGS. 30A and 30B are schematic views illustrating viewing angledependence of a conventional liquid crystal display device.

FIG. 31 is a schematic plan view of a conventional liquid crystaldisplay device in a wide viewing angle mode.

FIG. 32 is a cross-sectional view taken along the E-E′ line in FIG. 31.

FIG. 33 is a schematic view illustrating a method for producing theconventional liquid crystal display device shown in FIG. 31.

FIG. 34 is a schematic view illustrating the operation principle of aconventional liquid crystal display device in a wide viewing angle mode.

FIG. 35 is a schematic cross-sectional view of the conventional liquidcrystal display device in a wide viewing angle mode.

FIG. 36 is a schematic cross-sectional view of a conventional PALC.

FIG. 37 is a schematic view illustrating the operation principle of theconventional PALC.

FIG. 38 is a schematic view illustrating the difference in viewing anglein a large display device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the drawings. It should be noted that thepresent invention is not limited thereto.

Embodiment 1 Basic Operation

Referring to FIGS. 1A through 1D, the operation principle of a liquidcrystal display device 100 in an embodiment of the present inventionwill be described. FIG. 1A is a schematic cross-sectional view of theliquid crystal display device 100 when no voltage is being applied, andFIG. 1C is a schematic cross-sectional view thereof under theapplication of a voltage. FIG. 1B shows results obtained by observingthe upper surface of the liquid crystal display device 100 when novoltage is being applied with a polarizing microscope in crossed-Nicols,and FIG. 1D shows results obtained by observing the upper surface of theliquid crystal display device 100 under the application of a voltagewith a polarizing microscope in crossed-Nicols.

The liquid crystal display device 100 includes a liquid crystal layer 40containing an n-type liquid crystal material (liquid crystal molecules)42 with a negative dielectric anisotropy Δε between a pair of substrates32 and 34. Homeotropic alignment layers 38 a and 38 b are provided onthe surfaces of the substrates 32 and 34 in contact with the liquidcrystal layer 40. Convex portions 36 are formed on the surface of atleast one of the substrates 32 and 34 on the liquid crystal layer 40side. Because of the convex portions 36, the liquid crystal layer 40 hastwo different thicknesses d_(out) and d_(in). Consequently, a liquidcrystal region exhibiting axis-symmetrical alignment under theapplication of a voltage is defined as a region surrounded by the convexportions 36, as described later. In FIGS. 1A through 1D, electrodes forapplying a voltage to the liquid crystal layer 40 formed on thesubstrates 32 and 34 are omitted.

As shown in FIG. 1A, the liquid crystal molecules 42 are aligned by analignment regulating force of the homeotropic alignment layers 38 a and38 b in a direction vertical to the substrates 32 and 34 when no voltageis being applied. When pixel regions are observed when no voltage isbeing applied with a polarizing microscope in crossed-Nicols, a blackfield of view (normally black mode) is exhibited as shown in FIG. 1B.Upon the application of a voltage, the liquid crystal molecules 42having a negative dielectric anisotropy Δε are provided with a forcewhich aligns the major axes of the liquid crystal molecules 42 in adirection vertical to the electric field direction. Therefore, theliquid crystal molecules 42 are tilted from a direction vertical to thesubstrates 32 and 34 (gray-scale display state), as shown in FIG. 1C.When the pixel regions in this state are observed with a polarizingmicroscope in crossed-Nicols, extinction patterns are observed in thedirections of polarization axes as shown in FIG. 1D.

FIG. 2 shows a voltage-transmittance curve of the liquid crystal displaydevice 100 of the present invention. The abscissa axis represents avoltage applied to the liquid crystal layer 40, and the ordinate axisrepresents a relative transmittance. When a voltage is increased from anormally black state when no voltage is being applied, the transmittancegradually increases. A voltage at which the relative transmittance withrespect to a saturated transmittance becomes 10% is referred to asV_(th) (threshold voltage). When the voltage is further increased, thetransmittance further increases to reach saturation. A voltage at whichthe transmittance is saturated is referred to as V_(st) (saturationvoltage). In the case where a voltage applied to the liquid crystallayer 40 is between ½ V_(th) and V_(st), the transmittance reversiblychanges in the operation range shown in FIG. 2. Under the application ofa voltage in the vicinity of ½ V_(th), the liquid crystal molecules arealigned in a direction almost vertical to the substrates, whileremembering the symmetry with respect to central axes inaxis-symmetrical alignment. Thus, when a voltage exceeding ½ V_(th) isapplied, it is considered that the liquid crystal molecules reversiblyreturn to the “remembered” axis-symmetrical alignment state. However,when a voltage to be applied becomes lower than ½ V_(th), the liquidcrystal molecules are aligned in a direction almost vertical to thesubstrates without remembering the symmetry with respect to the centralaxes in the axis-symmetrical alignment. Thus, even when a voltageexceeding ½ V_(th) is applied again, the direction in which the liquidcrystal molecules are tilted is not uniquely determined. Therefore, dueto the presence of a plurality of central axes in axis-symmetricalalignment, the transmittance does not become stable. More specifically,a plurality of central axes are once formed in the regions defined bythe convex portions 36 (i.e., pixel regions). For example, at a stagewhere an n-type liquid crystal material is injected into a liquidcrystal cell, the liquid crystal molecules behave in the same way as inthe case of an applied voltage of less than ½ V_(th).

Thus, the display mode in the present embodiment becomes practicallyuseful by applying a voltage realizing axis-symmetrical alignment in theinitial display, and using the device in the range of a voltage at whichthe alignment is stable after the commencement of the display.

Convex Portions Defining Pixel Regions

As shown in FIG. 1A, the liquid crystal display device 100 of thepresent invention has convex portions 36 so as to surround the pixelregions. In the case where the thickness (cell gap) of the liquidcrystal layer 40 is uniform without convex portions 36, the positionsand sizes of liquid crystal domains (continuously aligned regions:regions with no disclination lines) are not defined. Therefore, theliquid crystal molecules are aligned in a random direction, resulting ina rough display in gray scales.

According to the present invention, the convex portions 36 define thepositions and sizes of the liquid crystal regions exhibitingaxis-symmetrical alignment. The convex portions 36 are formed for thepurpose of controlling the thickness of the liquid crystal layer 40 andweakening the interaction of the liquid crystal molecules between thepixel regions. Regarding the thickness of the liquid crystal layer 40,it is preferable that the thickness d_(out) of the liquid crystal layer40 on the periphery of the pixel region is smaller than the thicknessd_(in) of the liquid crystal layer 40 in the pixel region (openingportion), i.e., d_(in)>d_(out), and the relationship0.2×d_(in)≦d_(out)≦0.8×d_(in) is satisfied. More specifically, in thecase of 0.2×d_(in)>d_(out), the effect of weakening the interaction ofthe liquid crystal molecules between the pixel regions by the convexportions 36 is not sufficient, and it may be difficult to form a singleaxis-symmetrically aligned region in each pixel region. Furthermore, inthe case of d_(out)>0.8×d_(in), it may be difficult to inject a liquidcrystal material into a liquid crystal cell.

It is noted that a “pixel” is generally defined as the minimum unit forperforming a display. The term “pixel region” used herein refers to apartial region of a display device corresponding to the “pixel”. In thecase of pixels having a large aspect ratio (i.e., long pixels), aplurality of pixel regions may be formed with respect to one long pixel.The number of pixel regions formed corresponding to pixels is preferablyas small as possible, as long as the axis-symmetrical alignment isstably formed. The term “axis-symmetrical alignment” refers to, forexample, radial alignment, tangential alignment, etc.

Control of Positions of Central Axes in Axis-symmetrical Alignment

The positions of central axes in the axis-symmetrically aligned regionsgenerated under the application of a voltage have a great effect ondisplay quality. Referring to FIGS. 3A through 3D, the relationshipbetween the positions of central axes and the display quality will bedescribed. As shown in FIG. 3A, in the case where a central axis 44 ispositioned at the center of each pixel region, even when a displaysurface is observed with a cell tilted, all the pixel regions areobserved in the same way as shown in FIG. 3C. As shown in FIG. 3B, inthe case where the central axes 44 are positioned shifted away from thecenters of the pixel regions, the pixel regions with the shifted centralaxes are observed in a different way from the other pixel regions asshown in FIG. 3D, which results in a rough display. This problem becomesparticularly remarkable in gray scales.

The positions of central axes in axis-symmetrical alignment can becontrolled by adjusting the thickness d_(in)(x) of the liquid crystallayer in the pixel regions. As shown in FIGS. 4A and 4B, the thicknessd_(in)(x) of the liquid crystal layer is continuously changed so thatthe thickness d_(in) (x=0) of the liquid crystal layer becomes maximumand the thickness d_(in) (x=r) becomes minimum, where x=0 at the centerof the pixel region, and x=r at one end of the pixel region. It ispreferable that the differential coefficient of d_(in)(x) is alwaysnegative and continuous from x=0 to x=r. In view of the symmetry of theviewing angle characteristics, it is preferable that the thickness ofthe liquid crystal layer is as symmetric as possible with respect to thecenter of each pixel region.

The axis-symmetrical alignment is formed with good reproducibility bycontrolling the thickness of the liquid crystal layer as describedabove. This mechanism will be described with reference to FIGS. 5Athrough 5C. FIGS. 5A through 5C are schematic cross-sectional viewsillustrating a pixel region of the liquid crystal display device of thepresent invention.

As shown in FIG. 5A, a display electrode 52 is formed in a pixel regionon the surface of one substrate 32, and a homeotropic alignment layer 58a is formed so as to cover the display electrode 52. The homeotropicalignment layer 58 a has a cross-section in which the thickness d_(in)of the liquid crystal layer 40 changes as shown in FIGS. 4A and 4B. Thechanges in the thickness d_(f) of the homeotropic alignment layer 58 awith respect to a position x is opposite to the changes in the thicknessof the liquid crystal layer 40. Therefore, it is preferable that thedifferential coefficient of d_(f)(x) of the homeotropic alignment layer58 a is positive. A counter electrode 54 is formed on the surface of theother substrate 34 on the liquid crystal layer 40 side, and ahomeotropic alignment layer 58 b is formed so as to cover the counterelectrode 54. The homeotropic alignment layer 58 b has a flatcross-section.

Liquid crystal molecules 42 in the vicinity of the homeotropic alignmentlayer 58 a are aligned in a direction vertical to the surface of thehomeotropic alignment layer 58 a, so that they are tilted from thesubstrate surface. Thus, when a voltage is applied across the electrodes52 and 54, the major axes of the liquid crystal molecules 42 becometilted from an electrical field direction E. As a result, the liquidcrystal molecules 42 are tilted by the electrical field E only indirections represented by arrows in FIG. 5A. A tilt angle θ′ of theliquid crystal molecule from a direction normal to the substrate surfacepreferably satisfies the relationship 0<θ′≦3°. When θ′ exceeds about 3°,there is a large possibility that a phase difference may be caused bythe liquid crystal molecules and light may leak to decrease the contrastratio.

As described above, the cross-sectional shape (thickness) of thehomeotropic alignment layer is changed to vary the thickness of theliquid crystal layer 40 as described with reference to FIGS. 4A and 4B,whereby the positions of central axes in axis-symmetrical alignment canbe controlled, and the axis-symmetrical alignment can be realized withgood reproducibility.

In the example shown in FIG. 5A, although the thickness of the liquidcrystal layer 40 is controlled by the cross-sectional shape of thehomeotropic alignment layer 58 a, the method for controlling thethickness of the liquid crystal display device 40 is not limitedthereto. For example, as shown in FIG. 5B, a solid dielectric layer 59having a desired cross-section may be separately formed, and thehomeotropic alignment layer 58 a having a uniform thickness may beformed thereon. The solid dielectric layer 59 can be formed by using aconventionally used overcoat agent, more specifically, an epoxy-typecoating agent, an epoxyacrylate-type coating agent, and the like. In thepresent embodiment, the thickness of the thickest portion of the soliddielectric layer 59 is, for example, in the range of 500 to 10000 nm,and the thickness of the thinnest portion is, for example, in the rangeof 0 to 5000 nm.

In the case where the thickness of the liquid crystal layer 40 iscontrolled by using the solid dielectric layer 59, the solid dielectriclayer 59 is preferably formed on the display electrode 52. As shown inFIG. 5C, when the display electrode 52 is formed on the solid dielectriclayer 59, the electric field direction E is tilted from the substratesurface, so that in most cases, the direction in which the liquidcrystal molecules are tilted is not uniquely determined.

Liquid Crystal Material

The liquid crystal material used in the present invention is of ann-type which has a negative dielectric anisotropy (Δε<0). The absolutevalue of Δε can be appropriately determined depending upon the purpose.In general, considering that a driving voltage is decreased, theabsolute value is preferably large.

Retardation d·Δn under the application of a voltage is an importantfactor which influences critical device characteristics such astransmittance and viewing angle characteristics of a device. In thedisplay mode of the present invention, the retardation peculiar to aliquid crystal cell determined by the product of Δn peculiar to a liquidcrystal material and a thickness d of a liquid crystal layer is notnecessarily defined to be an optimum value. According to the presentinvention, the retardation at the maximum driving voltage to be used isimportant, which will be described below.

FIG. 6 shows a voltage-transmittance curve of a liquid crystal displaydevice having a retardation value larger than the optimum retardationvalue (first minimum condition under which a transmittance becomesmaximum: d·Δn=450 nm). In such a liquid crystal display device, it isnot required to use a voltage at which a transmittance exceeds themaximum point of a relative transmittance, and the device may be drivenin a region where the relative transmittance monotonously increases.More specifically, a voltage at which the relative transmittance becomesmaximum may be set as the maximum driving voltage V_(max) in FIG. 6.

Regarding the range of retardation, a product d·Δn (retardation) ofapparent Δn (anisotropy of a refractive index: a value at the maximumdriving voltage) of liquid crystal molecules when a liquid crystal cellis produced and an average thickness d of the liquid crystal layer ispreferably in the range of about 300 nm to about 500 nm. There is asecond minimum condition (retardation: about 1000 nm to about 1400 nm)for the transmittance to become local maximum. However, the secondminimum condition is not preferable since the viewing anglecharacteristics when no voltage is being applied decrease. Furthermore,the relationship between the level of an applied voltage and thetransmittance becomes inverted depending upon the viewing angle, what iscalled, a gray-scale inversion (contrast inversion) phenomenon occursunder the second minimum condition, which is not preferable.

The twist angle of the liquid crystal molecules in the liquid crystallayer is also an important factor determining the transmittance of theliquid crystal display device. According to the present invention, thetwist angle at the maximum driving voltage is as important as theretardation. In principle, the transmittance of the liquid crystaldisplay device becomes maximum in the case where the twist angle is 90°and 270°. However, in the case of the twist angle of 270°, it isdifficult to stably produce axis-symmetrical alignment, so that thetwist angle in the vicinity of 90° at which the transmittance becomesmaximum in the voltage-transmittance curve is preferable. The twistangle under the application of the maximum driving voltage is preferablyin the range of about 45° to about 110°. According to the presentinvention, since the n-type liquid crystal molecules are used, theapparent twist angle of the liquid crystal molecules depends upon avoltage. The twist angle when no voltage is being applied is almost 0°,and the twist angle increases with the increase in the applied voltage.When a sufficient voltage is applied, the twist angle approaches thatpeculiar to the liquid crystal material.

The combination of the twist angle and the retardation in theabove-mentioned range under the application of the maximum drivingvoltage is more preferable, because it allows the transmittance toapproach the maximum value more effectively.

Photocurable Resin

As described above with reference FIG. 2, it is preferable that avoltage of ½ V_(th) or more is always applied to the liquid crystaldisplay device of the present invention. If a voltage is applied toliquid crystal molecules aligned in a direction vertical to thesubstrates, the direction in which the liquid crystal molecules aretilted is not uniquely determined. As a result, a plurality of centralaxes are transiently formed. If a voltage is continued to be applied, asingle central axis is formed in each region defined by the convexportions, and this state is stably maintained as long as a voltage of ½V_(th) or more is applied.

An axis-symmetrical alignment fixing layer is formed by curing aphotocurable resin mixed in a liquid crystal material under theapplication of a voltage of ½ V_(th) or more for stabilizingaxis-symmetrical alignment. The axis-symmetrical alignment fixing layeris capable of stabilizing the axis-symmetrical alignment. After thephotocurable resin is cured, a plurality of central axes are not formedeven when a voltage of ½ V_(th) or more is removed. Thus, theaxis-symmetrical alignment is formed with good reproducibility. Theaxis-symmetrical fixing layer will be described in detail later.

As the photocurable resin used in the present invention, an acrylatetype resin, a methacrylate type resin, a styrene type resin, andderivatives thereof can be used. By adding a photopolymerizationinitiator to these resins, the photocurable resin can be cured moreefficiently. A thermosetting resin can also be used.

The adding amount of the curable resin (photocurable or thermosettingresin) is not particularly limited in the present invention, with theoptimum amount being variable depending upon the material. However, itis preferable that the content of the resin (% based on the total weightincluding the weight of the liquid crystal material) is about 0.1% toabout 5%. When the content is less than about 0.1%, the axis-symmetricalalignment state cannot be stabilized by the cured resin. When thecontent exceeds about 5%, the effect of the homeotropic alignment layeris reduced, so that the liquid crystal molecules are aligned largelyshifted from homeotropic alignment when no voltage is being applied.This causes the light transmittance (light leakage) to increase,deteriorating the black state when no voltage is being applied.

Phase Difference Plate

In the case where a vertically aligned liquid crystal molecules aredisposed between two polarizing plates whose optical axes are orthogonalto each other, a satisfactory black state with a high contrast isobtained in the front surface direction. However, when the device isobserved from a different viewing angle, a contrast ratio is decreaseddue to light leakage, depending upon (i) the viewing angle dependence ofcharacteristics of the polarizing plates and (ii) the viewing angledependence of retardation of a liquid crystal layer (the retardation ofthe vertically aligned liquid crystal molecules is changed dependingupon the direction). This phenomenon occurs particularly in the 45°direction from the polarization axis (azimuth angle, i.e.,intra-substrate angle). In order to prevent this phenomenon, it iseffective to decrease the retardation of the vertically aligned liquidcrystal molecules. Alternatively, it is preferable that a phasedifference plate having a negative uniaxial “Frisbee-type” refractiveoval body is disposed between the liquid crystal cell and the polarizingplate. A biaxial phase difference film having the relationship in whichthe refractive index n_(x,y) in an intra-display surface direction isgreater than the refractive index n_(z) in a direction vertical to adisplay surface may be used. It is preferable that the phase differenceof this phase difference plate is smaller than the retardation peculiarto the liquid crystal cell determined by the product of Δn peculiar tothe liquid crystal material and a thickness d of the liquid crystallayer. More preferably, the retardation peculiar to the liquid crystalcell is in the range of about 30% to about 80%. When the retardation isless than about 30%, the effect of the phase difference plate is small.When the retardation is more than about 80%, staining becomes large inthe wide viewing angle direction, which is not preferable.

Homeotropic Alignment Layer

As the homeotropic alignment layer, any layers having the surfacecapable of vertically aligning liquid crystal molecules may be used. Thehomeotropic alignment layer can be made of an inorganic material or anorganic material. For example, polyimide-type materials (JALS-204,produced by Japan Synthetic Rubber Co., Ltd.; 1211, produced by NissanChemical Industries, Ltd.), inorganic materials (EXP-OA003; produced byNissan Chemical Industries, Ltd.), and the like can be used.

Embodiment 2

The present invention is also applicable to a PALC. FIG. 7 is aschematic cross-sectional view of a PALC 400 in the present embodiment.The PALC 400 includes a counter substrate 120, a plasma substrate 110,and a liquid crystal layer 102 disposed therebetween. The liquid crystallayer 102 is sealed with a sealant 106. The plasma substrate 110includes a substrate 111, a dielectric sheet 116 opposing the substrate111, and a plurality of plasma chambers 113 defined by partition walls112 provided between the substrate 111 and the dielectric sheet 116. Theplasma chambers 113 oppose the liquid crystal layer 102 with thedielectric layer 116 disposed therebetween. Gas sealed in each plasmachamber 113 is ionized by applying a voltage across an anode 114 and acathode 115 formed on the surface of the substrate 111 on the plasmachamber 113 side, whereby plasma discharge occurs. A plurality ofchambers 113 extend in the shape of stripes in a direction vertical tothe drawing surface of FIG. 7 in such a manner as to be orthogonal totransparent electrodes 105 formed on the surface of the countersubstrate 120 on the liquid crystal layer 102 side. Intersections of theplasma chambers 113 and the transparent electrodes 105 define pixelregions. Compared with a simple matrix type liquid crystal displaydevice, the transparent electrodes 105 on the counter substrate 120correspond to display electrodes (signal electrodes), and the plasmachambers 113 correspond to scanning electrodes.

Convex portions 132 in the shape of a lattice are formed on the countersubstrate 120 on the liquid crystal layer 102 side so as to correspondto the non-pixel regions. The convex portions 132 allowaxis-symmetrically aligned regions to be formed so as to correspond tothe pixel regions. Furthermore, homeotropic alignment layers 134 a and134 b are provided on the surfaces of the plasma substrate 110 and thecounter substrate 120 on the liquid crystal layer 102 side.

The basic operation, the convex portions defining the pixel regions, thecontrol of the positions of central axes in axis-symmetrical alignment,the liquid crystal material, the photocurable resin, the phasedifference plate, and the homeotropic alignment layer are basically thesame as described in Embodiment 1. Therefore, the detailed descriptionsthereof will be omitted here. The unique characteristics of the PALCwill be described below.

In the case of the PALC according to the present invention, regarding Δεof the liquid crystal material, ε_(//) is preferably as small aspossible because voltage can be easily applied to the liquid crystallayer. More specifically, ε_(//) is preferably in the range of 2.5 to3.3. (Here, Δε is defined as a difference between ε_(//) and ε_(⊥).ε_(//) is the component of the dielectric constant parallel to thedirection of orientation vector of the liquid crystal molecules, andε_(⊥)is the component of the dielectric constant perpendicular thereto.)

Regarding the solid dielectric layer, a voltage to be applied to theliquid crystal layer 102 is divided between the liquid crystal layer 102and the dielectric sheet 116 in accordance with the capacitance (seeFIG. 7). In general, in the case of the PALC, the thickness of thedielectric sheet 116 is larger than that of the liquid crystal layer102, so that a voltage applied to the liquid crystal layer 102 issmaller than that applied to the dielectric sheet 116. Thus, the effectof the voltage drop caused by the formation of a solid dielectric layeron the surface of the dielectric sheet 116 on the liquid crystal layer102 side is relatively small, so that the formation of a soliddielectric layer with a thickness of about several μm does not cause anypractical problems.

Arrangement of Polarizing Plates

When there is a difference in refractive index on an attachment surfacebetween the plasma substrate and the thin glass sheet (i.e., thedielectric sheet), light leaks from the attachment surface due to thebirefringence and the difference in refractive index with respect topolarized light, whereby the attachment portion becomes visible. Thisphenomenon becomes most obvious in the case where the angle between thepolarization axes of the polarizing plates and the surface having thedifference in refractive index is 45°. In the case where this angle is0° or 90°, this phenomenon becomes minimum. In the case of a device in aTN mode, in order to widen the viewing angle in the sideward directionas seen by an observer, considering its viewing angle characteristics(FIG. 8A), the polarizing plates are generally disposed in such a mannerthat the polarization axes are tilted by 45° from the crosswisedirection on the display surface as shown in FIG. 8B. When thepolarizing plates of the PALC in a TN mode are disposed in this way,since the plasma chamber structure causing the difference in refractiveindex extends in the ordinate or abscissa direction of the displaysurface, the plasma chamber structure is easily visualized. However, theaxis-symmetrical alignment mode (vertical ASM mode) used in the presentinvention has viewing angle characteristics with high symmetry, as shownin FIG. 9; therefore, the polarization axes of the polarizing plates canbe disposed in a crosswise direction of the display surface, whereby theplasma chamber structure can be made invisible. In this respect, thereis an advantage that the axis-symmetrical alignment is applied to thePALC.

Embodiment 3 Basic Structure and Operation Principle

In the present embodiment, the case where concave portions orthrough-holes (hereinafter, referred to as axis-symmetrical alignmentcentral axis forming portions) for axis-symmetrically aligning liquidcrystal molecules are provided at predetermined positions (preferably,substantially central portions of the pixel regions) of electrodes on atleast one substrate will be described.

Referring to FIGS. 10A through 10D, the basic structure and operationprinciple of a liquid crystal display device 100 in the presentembodiment will be described. FIG. 10A is a schematic cross-sectionalview of the liquid crystal display device 100 when no voltage is beingapplied, and FIG. 10C is a schematic cross-sectional view thereof underthe application of a voltage. FIG. 10B shows results obtained byobserving the upper surface of the liquid crystal display device 100shown in FIG. 10A with a polarizing microscope in crossed-Nicols, andFIG. 10D shows results obtained by observing the upper surface of theliquid crystal display device 100 shown in FIG. 10B with a polarizingmicroscope in crossed-Nicols.

The liquid crystal display device 100 includes a liquid crystal layer 40containing a liquid crystal material (liquid crystal molecules) 42 witha negative dielectric anisotropy Δε between a pair of substrates 32 and34. Transparent electrodes 31 and 33 are provided on the surfaces of thesubstrates 32 and 34 on the liquid crystal layer 40 side, respectively.Homeotropic alignment layers 38 a and 38 b are provided on thetransparent electrodes 31 and 33, respectively. Furthermore, anaxis-symmetrical alignment central axis forming portion 35 is providedat a predetermined position (preferably, a substantially central portionof each pixel region) of each of the electrodes (electrodes 31 in FIG.10A) on at least one substrate. Convex portions 36 are formed on thesurface of at least one of the substrates 32 and 34 (substrate 32 inFIG. 10A) on the liquid crystal layer 40 side.

Because of the convex portions 36, the liquid crystal layer 40 has twodifferent thicknesses d_(out) and d_(in). As a result, upon theapplication of a voltage for forming axis-symmetrical alignment centralaxes (described later), liquid crystal regions exhibitingaxis-symmetrical alignment are defined by the convex portions 36. Theformation of the convex portions 36 defines the positions and sizes ofthe liquid crystal regions exhibiting axis-symmetrical alignment. Thedetail of the convex portions 36 is as described in Embodiment 1.Furthermore, the position of each axis-symmetrical alignment centralaxis is controlled by the axis-symmetrical alignment central axisforming portion 35. Thus, as shown in FIG. 10C, the liquid crystalmolecules 42 are axis-symmetrically aligned around an axis-symmetricalalignment central axis 44 formed in the axis-symmetrical alignmentcentral axis forming portion 35 in the pixel region defined by theconvex portions 36.

The liquid crystal molecules 42 are aligned in a direction vertical tothe substrates 32 and 34 by an alignment regulating force of thehomeotropic alignment layers 38 a and 38 b when no voltage is beingapplied as shown in FIG. 10A. When the pixel regions are observed with apolarizing microscope in crossed-Nicols when no voltage is beingapplied, a dark field of view (normally black mode) is exhibited asshown in FIG. 10B. Upon the application of a voltage, the liquid crystalmolecules 42 having a negative dielectric anisotropy Δε are providedwith a force which aligns the major axes of the liquid crystal molecules42 in a direction vertical to the electric field direction. Therefore,the liquid crystal molecules 42 are tilted from the direction verticalto the substrates as shown in FIG. 10B (gray-scale display state). Whenthe pixel regions in this state are observed with a polarizingmicroscope in crossed-Nicols, extinction patterns are observed in thedirections of polarization axes.

FIG. 2 shows a voltage-transmittance curve of the liquid crystal displaydevice of the present invention. The abscissa axis represents a voltage,and the ordinate axis represents a relative transmittance. As shown inFIG. 2, when a voltage is increased, the transmittance graduallyincreases. When the voltage is further increased, the transmittancefurther increases to reach saturation.

When a voltage is increased from the non-application state, the liquidcrystal molecules 42 are tilted from a direction vertical to thesubstrates 32 and 34. However, the direction in which the liquid crystalmolecules 42 are tilted is not uniquely determined. According to thepresent invention, because of the convex portions 36, a plurality ofcentral axes in axis-symmetrical alignment (hereinafter, merely referredto as “central axes”) are formed in liquid crystal regions exhibitingaxis-symmetrical alignment defined by the convex portions 36. However,when such a plurality of central axes are present, both the alignmentand the transmittance are unstable.

When a voltage of ½ V_(th) or more is continued to be applied, aplurality of central axes become a single central axis in each liquidcrystal region defined by the convex portions 36. In the case where avoltage applied to the liquid crystal layer 40 is between ½ V_(th) andV_(st), the transmittance reversibly changes in the operation range asshown in FIG. 2. Under the condition that a voltage in the vicinity of ½V_(th) is applied, the liquid crystal molecules are aligned in adirection almost vertical to the substrates, while remembering theaxis-symmetrical alignment state under the application of a voltage of ½V_(th) or more, i.e., the symmetry with respect to the central axis.However, when the voltage is removed or the voltage is decreased to lessthan ½ V_(th), the liquid crystal molecules are aligned in a directionalmost vertical to the substrates and return to a state not rememberingthe axis-symmetrical alignment state. Thus, even when a voltageexceeding ½ V_(th) is applied again, a plurality of central axes areonce again formed. For example, at a stage where an n-type liquidcrystal material is injected into a liquid crystal cell, the liquidcrystal molecules behave in the same way as in the case of an appliedvoltage of less than ½ V_(th).

As described above, the liquid crystal display device of the presentinvention operates in a normally black mode in which the liquid crystalmolecules are aligned in a direction vertical to the substrates toperform a black display when no voltage is being applied, and the liquidcrystal molecules are axis-symmetrically aligned around a central axisformed in each pixel region to perform a white display under theapplication of a voltage. However, a plurality of central axes areformed after the application of a voltage, so that the operation becomesunstable with a black display being performed when no voltage is beingapplied. In order to achieve a stable operation in the display mode ofthe present invention, it is desirable that one central axis is formedin each pixel region prior to a display operation.

In order to form one central axis in each pixel region before thedisplay operation, a predetermined voltage, i.e., a voltage of ½ V_(th)or more should be applied. Thus, one central axis is formed in eachpixel region, whereby a stable axis-symmetrical alignment state can berealized during a white display. However, the removal of the voltageallows a plurality of central axes to be formed as in an initialunstable state. Therefore, the device should be used under theapplication of a predetermined voltage, i.e., a voltage in the vicinityof ½ V_(th) without removing the voltage even during a black displayafter the commencement of a display. In the display mode of the presentinvention, the device is preferably used in the range of a voltage atwhich a stable axis-symmetrical alignment state is obtained, i.e., inthe range of ½ V_(th) to V_(st).

To form one central axis in each pixel region before the displayoperation for the purpose of obtaining a stable operation state isreferred to as “axis-symmetrical alignment central axis formingprocess”. A voltage applied for the purpose of forming central axes isreferred to as “axis-symmetrical alignment central axis formingvoltage”.

Control of the Positions of Central Axes

As described above, according to the present invention, the liquidcrystal molecules are aligned in a direction vertical to the substrateswhen no voltage is being applied. When a voltage is continued to beapplied, the liquid crystal molecules are axis-symmetrically alignedaround one central axis in each liquid crystal region defined by theconvex portions. Thus, a liquid crystal display device with a highcontrast and a wide viewing angle can be realized.

However, since the direction in which the liquid crystal molecules aretilted under the application of a voltage is not uniquely determined,the central axes can be formed at arbitrary positions, depending uponthe pixel region. For example, there is a possibility that the centralaxis is formed at different positions even in the identical pixel regionevery time a voltage is applied. Alternatively, there is a possibilitythat even if an identical voltage is simultaneously applied, anaxis-symmetrical alignment central axis forming voltage may be appliedto the liquid crystal molecules in various manners depending upon thepixel region, whereby the central axes are formed at differentpositions, depending upon the pixel region.

When the positions at which the central axes are formed vary dependingupon the pixel region, there is a great effect on display quality. Therelationship between the positions of the central axes and the displayquality is as described with reference to FIGS. 3A through 3D. Morespecifically, in the case where the central axis 44 is formed at eachcentral position in the pixel regions as shown in FIG. 3A, all the pixelregions are observed in a similar manner even when the display surfaceis observed with a cell tilted as shown in FIG. 3C. In the case wheresome central axes are formed shifted from central portions of the pixelregions as shown in FIG. 3B, the pixel regions with the central axesshifted are observed in a different manner from the other pixel regionsas shown in FIG. 3D, so that a nonuniform (rough) display is obtained.This problem becomes serious particularly in a gray-scale display.

In order to obtain a display without any roughness, it is preferablethat the positions of the central axes are controlled by conducting anaxis-symmetrical alignment central axis forming process prior toperforming a display. Regions where the liquid crystal molecules keep ahomeotropic alignment state even under the application of a voltage areprovided in the pixel regions by the axis symmetrical alignment centralaxis forming process, whereby the positions of the central axes can becontrolled. The regions where the liquid crystal molecules keep ahomeotropic alignment state even under the application of a voltage canbe provided by forming axis-symmetrical alignment central axis formingportions in the electrodes in the pixel regions. In this case, it ispreferable that Sa satisfies 0%<Sa/A<about 4%, where Sa is an area of aregion where the liquid crystal molecules are aligned in a directionvertical to the substrates under the application of an axis-symmetricalalignment central axis forming voltage in each pixel region, and A is anarea of each pixel region, for the following reason. When Sa is 0, thereis no effect of controlling the positions of the central axes. When Sais about 4% or more, the ratio of the axis-symmetrical alignment centralaxis forming portions which do not contribute to a display is too large,and those portions become black defects, decreasing the contrast in mostcases.

The liquid crystal molecules in the axis-symmetrical alignment centralaxis forming portions are stable without the alignment state thereofbeing influenced by an electric field. Furthermore, even when a centralaxis is formed in a position of the pixel region other than the portionwhere the liquid crystal molecules keep a homeotropic alignment stateeven under the application of a voltage, the central axis moves from theportion where it is originally formed to the portion where the liquidcrystal molecules keep a homeotropic alignment state by continuing toapply an axis-symmetrical alignment central axis forming voltage. Thus,the central axis is formed in the portion of the pixel region where theliquid crystal molecules keep a homeotropic alignment state even underthe application of a voltage. The time required to allow the centralaxis to move to a predetermined position (i.e., a portion where theliquid crystal molecules keep a homeotropic alignment state even underthe application of a voltage) should be prescribed to be, for example,tens of seconds or more. Furthermore, the application of anaxis-symmetrical alignment central axis forming voltage while heating aliquid crystal cell facilitates the movement of the central axis fromthe portion where the axis is originally formed to the portion where theliquid crystal molecules keep a homeotropic alignment state, as a resultof which the controllability of the positions of the central axes arefurthermore improved.

Alternatively, by providing an axis-symmetrical alignment central axisforming portion at a predetermined position (preferably, a substantiallycentral portion of each pixel region) of each electrode in the pixelregions, the positions of the central axes can be controlled. FIGS. 11Aand 11B show the states of the electric force and the alignment of theliquid crystal molecules under the application of a voltage to a liquidcrystal cell in which an axis-symmetrical alignment central axis formingportion is provided in a pixel region. In these figures, the referencenumeral 1 denotes substrates, 2 denotes electrodes, 2 a denotes anaxis-symmetrical alignment central axis forming portion, 13 denotes anelectric force, and 14 denotes liquid crystal molecules.

An electric field in the vicinity of the boundary between theaxis-symmetrical alignment central axis forming portion 2 a and theelectrode 2 is strained by providing the axis-symmetrical alignmentcentral axis forming portion 2 a, and as shown in FIG. 11A, an electricforce 13 having a component parallel to the substrates is generated.Consequently, as shown in FIG. 11B, the liquid crystal molecules in thepixel region are influenced by the strained electric field, and evenwhen the central axis is formed in a portion of the pixel region notcorresponding to the axis-symmetrical alignment central axis formingportion 2 a, the central axis moves from the portion where the axis isoriginally formed to a portion of the pixel region corresponding to theaxis-symmetrical alignment central axis forming portion 2 a. Thus, theaxis-symmetrical alignment central axis is formed in the portion of thepixel region corresponding to the axis-symmetrical alignment centralaxis forming portion 2 a.

Alternatively, the positions of the central axes can be controlled byadjusting the thickness of the liquid crystal layer in the pixel region.The adjustment of the thickness of the liquid crystal layer in the pixelregion is as described in Embodiment 1 with reference to FIGS. 4A and4B.

Stabilization of an Axis-symmetrical Alignment State of the LiquidCrystal Molecules Under the Application of an Axis-symmetrical AlignmentCentral Axis Forming Voltage

In order to perform a stable display operation in a display mode of thepresent invention, it is desirable that an axis-symmetrical alignmentstate is stabilized by forming one central axis in each pixel regionprior to a display operation. For this purpose, as described above, theaxis-symmetrical alignment central axis forming process should beconducted, in which a predetermined voltage is applied prior to thedisplay operation. Furthermore, it is preferable that a predeterminedvoltage is applied even during a black display after the commencement ofthe display operation, and the operation voltage, for example, in therange of ½ V_(th) to V_(st), capable of obtaining a stableaxis-symmetrical alignment state is used. The reason for applying apredetermined voltage even during a black display is that the liquidcrystal molecules are allowed to remember an axis-symmetrical alignmentstate (i.e., symmetry with respect to a central axis) formed upon theapplication of a voltage of ½ V_(th) or more so as not to return to theinitial state. The axis-symmetrical alignment central axis formingprocess may be conducted every time before the commencement of thedisplay operation after the completion of a liquid crystal displaydevice, or may be included in the course of the production of a liquidcrystal display device.

Axis-symmetrical Alignment Fixing Layer

According to the present invention, when no voltage is being applied,the liquid crystal molecules may be prescribed to assume anaxis-symmetrical alignment state similar to that under the applicationof a voltage in the vicinity of ½ V_(th). In order to realize this, theaxis-symmetrical alignment fixing layer can be formed on the surface ofat least one of the substrates on the liquid crystal layer side. Byforming the axis-symmetrical alignment fixing layer, an axis-symmetricalpretilt angle can be provided to liquid crystal molecules in each liquidcrystal region exhibiting axis-symmetrical alignment even under thecondition that a voltage of ½ V_(th) or more is not applied. Althoughthe liquid crystal molecules are provided with a pretilt angle by theaxis-symmetrical alignment fixing layer even when no voltage is beingapplied, the tilt of the liquid crystal molecules from a directionnormal to the substrates is small, and a black level is substantiallyequal to that in the case without the axis-symmetrical alignment fixinglayer.

The axis-symmetrical alignment fixing layer can be formed by a methodincluding the steps of disposing a precursor mixture containing at leasta liquid crystal material and a photocurable material between a pair ofsubstrates, and curing the photocurable material in the mixture. Thephotocurable material is cured, for example, by exposing the precursormixture disposed between the substrate to light under the application ofan axis-symmetrical alignment central axis forming voltage. Anyappropriate light exposure conditions can be adopted. A thermosettingmaterial can be used in place of the photocurable material. In the caseof using the thermosetting material, any appropriate curing conditions(heating conditions) can be adopted. The content of the curable materialin the precursor mixture is as described in Embodiment 1.

The photocurable material is preferably used for the following reason.Desired regions of the photocurable material can be selectively cured,using a photomask or the like, so that liquid crystal regions (polymerregions) are likely to be formed in a regular manner in terms of space.If materials transmitting light with a desired wavelength are used fortransparent electrodes and color filters in a liquid crystal displaydevice, these members can be used in place of a photomask. The use ofthe members of the liquid crystal display device as a photomask has theadvantage in that the liquid crystal regions can be formed in aself-matching manner.

In order for the axis-symmetrical alignment fixing layer to provide anaxis-symmetrical pretilt angle to the liquid crystal molecules in eachliquid crystal region exhibiting axis-symmetrical alignment when novoltage of ½ V_(th) or more is being applied, it is desirable that theliquid crystal molecules are tilted at a certain angle with respect to adirection normal to the substrates in the course of the formation of theaxis-symmetrical alignment fixing layer (that is, it is desirable thatthe liquid crystal molecules have a tilt angle). In order to tilt theliquid crystal molecules at a certain angle with respect to a directionnormal to the substrates, a voltage should be applied. The appliedvoltage should be, for example, in the range of ½ V_(th) to V_(st)capable of stabilizing axis-symmetrical alignment.

The axis-symmetrical alignment central axis forming voltage can beapplied by using the electrodes (31 and 33 in FIG. 10A) which apply avoltage to the liquid crystal layer 40 for performing a display. Theaxis-symmetrical alignment central axis forming voltage is preferably anAC with a frequency of 1 Hz or more. The reason for using an AC is thatthe use of a DC may degrade the precursor mixture. When the frequency ofthe voltage is less than 1 Hz, the liquid crystal molecules becomeunlikely to follow the changes in voltage, making it impossible toaxis-symmetrically align the liquid crystal molecules. In order to tiltthe liquid crystal molecules at a certain angle with respect to adirection normal to the substrates, a magnetic field may be applied inplace of the axis-symmetrical alignment central axis forming voltage.

EXAMPLES

Hereinafter, the present invention will be described by way ofillustrative examples. However, the present invention is not limitedthereto.

Example 1

Referring to FIGS. 12A and 12B, a method for producing a liquid crystaldisplay device in the present example will be described. Convex portions66 with a height of about 3 μm were formed with a photoresist (OMR83;produced by Tokyo Ohka-sha) on regions other than pixel regions of asubstrate 62 having transparent electrodes 63 made of ITO (thickness:about 100 nm) on its surface. Then, spacers 65 with a height of about 5μm were formed on the convex portions 66 with photosensitive polyimide.The size of a region (i.e., a pixel region) defined by the convexportions 66 was prescribed to be 100 μm×100 μm. Polyimide (JALS-204;produced by Japan Synthetic Rubber Co., Ltd.) was spin-coated onto theresultant substrate to form a homeotropic alignment layer 68.Furthermore, a homeotropic alignment layer was also formed with the samematerial on transparent electrodes of the other substrate (not shown).These substrates were attached to each other to complete a liquidcrystal cell.

An n-type liquid crystal material (Δε=−4.0; Δn=0.08; a twist anglepeculiar to the liquid crystal material=90° in a cell gap of 5 μm,) wasinjected into the cell produced as described above, and a voltage ofabout 7 volts was applied to the cell. Immediately after the applicationof the voltage, a plurality of central axes are present in an initialstate. When the voltage is continued to be applied, one axis-symmetricalalignment region (monodomain) was formed in each pixel region.

Polarizing plates were disposed in crossed-Nicols on both sides of thecell, whereby a liquid crystal display device was produced. Thestructure of the liquid crystal display device thus obtained wassubstantially the same as that of the liquid crystal display device 100shown in FIGS. 1A through 1D, except that the cross-section of thehomeotropic alignment layer 68 had the shape of a mortar as shown inFIG. 12A (polarizing plates are not shown). Since the homeotropicalignment layer 68 has a cross-section in the shape of a mortar, adifferential coefficient of a curve showing changes in thickness withrespect to the position (from a central portion of a pixel to aperipheral portion thereof) is positive, and a differential coefficientof a curve showing changes in thickness of the liquid crystal layer inthe pixel region is negative.

The axis-symmetrical alignment of the cell in Example 1 is stable underthe application of a voltage of ½ V_(th) or more, and is disturbed whenthe voltage is decreased to less than ½ V_(th) to return to an initialstate. When a voltage is applied to the cell again, an initialaxis-symmetrical alignment with a plurality of central axes is obtained.Thereafter, an axis-symmetrical alignment state in which one centralaxis is formed in each pixel region is obtained. This phenomenon isobtained even when the same experiment is conducted 20 times. Afterforming the axis-symmetrical alignment state by the application of avoltage of ½ V_(th) or more, the cell in Example 1 was measured forelectro-optic characteristics in a voltage range (½ V_(th) or more) inwhich the axis-symmetrical alignment was stable.

FIG. 13 shows the electro-optic characteristics thus obtained. As isapparent from FIG. 13, the liquid crystal display device of the presentinvention had a satisfactory contrast ratio (CR=300:1, 5 volts) with alow transmittance when no voltage is being applied. Regarding theviewing angle characteristics, a high contrast ratio was obtained in awide viewing angle range as shown in FIG. 14. In FIG. 14, ψ representsan azimuth angle (i.e., intra-display surface angle), θ represents aviewing angle (i.e., a tilt angle from a normal to the display surface),and the hatched portion represents the region with a contrast of 10:1 ormore.

Comparative Example 1

In Comparative Example 1, the homeotropic alignment layer 68 wasdirectly formed on the transparent electrode 63 formed on the surface ofthe substrate 62 shown in FIG. 12A. Thereafter, the spacers 65 wereformed using photosensitive polyimide in the same way as in Example 1.In Comparative Example 1, the convex portions 66 as shown in FIGS. 12Aand 12B were not formed. The resultant substrate was attached to thecounter substrate obtained in the same way as in Example 1 to produce aliquid crystal cell. The thickness of the liquid crystal layer in thepixel regions in the cell was constant.

When the same material as that in Example 1 was injected into the cell,the liquid crystal molecules were randomly aligned, and disclinationlines were formed in a random manner. The cell was observed under theapplication of a voltage, showing that the display was rough in grayscales.

Example 2

As shown in FIG. 15, a projection 69 was formed in a central portion ofa pixel region on the substrate 62 having the convex portions 66 inExample 1 using a resist material (OMR83). The width of the projection69 is preferably about {fraction (1/10)} or less of that of the pixelregion. When the width of the projection 69 exceeds about {fraction(1/10)} of that of the pixel region, the opening ratio decreases,resulting in a decrease in transmittance in a device, which is notpreferable. A liquid crystal cell was produced in the same way as inExample 1, except for providing the projections 69.

As a result of the observation of the cell, a central axis was formed ata position of each projection 69, and thus, a liquid crystal displaydevice in which central axes were formed at central portions in almostall the pixel regions was obtained. When the liquid crystal displaydevice was observed in various viewing angle directions, a displaywithout roughness was obtained.

Examples 3 and 4 and Comparative Examples 2 and 3

Liquid crystal display devices in Examples 3 and 4 and ComparativeExamples 2 and 3 were produced in the same way as in Example 1, exceptthat a cell gap (thickness of a liquid crystal layer) was adjusted asshown in Table 1. The adding amount of a chiral agent (S-811: producedby Merck & Co., Inc.) in the liquid crystal material used in each liquidcrystal display device was adjusted in such a manner that the twistangle peculiar to the liquid crystal material became 90°.

TABLE 1 Comparative Example Example 1 3 4 2 3 Cell gap d (μm) 5 4.4 5.63.1 6.5 d · Δn (nm) at V_(max) 400 352 448 248 520    Transmittance (%)70 58 73 39 72**  at V_(max)* *Relative value with a transmittance inparallel Nicols being 100% **Measured at a maximum transmittance

When an increasing voltage was applied to the liquid crystal cell inComparative Example 3 with a retardation of 520 nm until a maximumtransmittance was obtained, a transmittance decreased, and a contrastinversion phenomenon (i.e., a phenomenon occurring when the voltageexceeds V_(max) in FIG. 6) was observed in a gray-scale display. In theliquid crystal display device in Comparative Example 2 with aretardation of less than 300 nm, a transmittance was low. It isunderstood from the experimental results shown in Table 1 that theproduct d·Δn (retardation) of Δn (birefringence at the maximum drivingvoltage) of a liquid crystal material and an average thickness d of aliquid crystal layer is preferably in the range of about 300 to about500 nm.

Examples 5 and 6 and Comparative Examples 4 and 5

Liquid crystal display devices having different twist angles as shown inTable 2 (Examples 5 and 6 and Comparative Examples 4 and 5) wereproduced by adjusting the adding amount of a chiral agent (S-811:produced by Merck & Co., Inc.) in the liquid crystal material used inthe liquid crystal display device in Example 1. The electro-opticcharacteristics of the liquid crystal display devices were measuredunder the application of a voltage at which the transmittance of eachdevice became maximum.

TABLE 2 Comparative Example Example 1 5 6 4 5 Twist angle (°) 90 50110   30 120   Transmittance (%) 70 41 50** 35 35** at V_(max)**Relative value with a transmittance in parallel Nicols being 100% **Acontrast inversion phenomenon occurs when a voltage exceeding a value atwhich a transmittance becomes maximum is applied

It is understood from the results shown in Table 2 that the twist angleunder the application of the maximum driving voltage is preferably inthe range of 45° to 110°.

Example 7

A phase difference plate (retardation: 150 nm ascribable to n_(x)=n_(y),n_(x)−n_(z)) having a “Frisbee-type” refractive oval body was placed onone side of the liquid crystal display device in Example 1. FIG. 16shows the results obtained by measuring the viewing anglecharacteristics of the liquid crystal display device. It is understoodfrom FIG. 16 that the viewing angle of the liquid crystal display devicein Example 7 was wider than that of the liquid crystal display device inExample 1 (see FIG. 14).

Example 8

In the present example, a method for stabilizing axis-symmetricalalignment of liquid crystal molecules by curing a photocurable resinmixed in a liquid crystal material (i.e., by forming an axis-symmetricalalignment fixing layer) will be described.

FIG. 17 shows a schematic partial cross-sectional view of a liquidcrystal display device in Example 8. A liquid crystal display device 200includes a liquid crystal layer 80 containing an n-type liquid crystalmaterial (liquid crystal molecules) 92 having a negative dielectricanisotropy Δε between a pair of substrates 82 and 84. Homeotropicalignment layers 88 a and 88 b are provided on the surfaces of thesubstrates 82 and 84 on the liquid crystal layer 80 side. Convexportions 86 are provided on the surface of at least one of thesubstrates 82 and 84 on the liquid crystal layer 80 side. The liquidcrystal layer 80 has two different thicknesses because of the convexportions 86. Consequently, as described above, liquid crystal regionsexhibiting axis-symmetrical alignment are defined by the convex portions86 under the application of a voltage. In FIG. 17, electrodes formed onthe substrates 82 and 84 for applying a voltage to the liquid crystallayer 80 are omitted. The liquid crystal display device 200 is differentfrom the liquid crystal display device 100 in Example 1 in thataxis-symmetrical alignment fixing layers 90 a and 90 b are formed on thehomeotropic alignment layers 88 a and 88 b. The axis-symmetricalalignment fixing layers 90 a and 90 b allow the liquid crystal moleculesin the pixel regions to keep axis-symmetrical alignment even when novoltage is being applied. Therefore, even when a voltage of less than ½V_(th) is applied (or a voltage is not applied) for driving the liquidcrystal display device 200, the electro-optic characteristics as shownin FIG. 2 can be obtained with good reproducibility. Theaxis-symmetrical alignment fixing layers 90 a and 90 b keepingaxis-symmetrical alignment (pretilt) of the liquid crystal molecules areformed by curing a curable resin mixed in a liquid crystal materialunder the application of a voltage of ½ V_(th) or more to the liquidcrystal layer.

Hereinafter, referring to FIG. 18, a method for producing the liquidcrystal display device 200 will be described in detail. Convex portions66 with a height of about 2.5 μm were formed with a photoresist (OMR83;produced by Tokyo Ohka-sha) on regions other than pixel regions of asubstrate 62 having transparent electrodes 63 made of ITO (thickness:about 100 nm) on its surface. Then, spacers 65 with a height of about 5μm were formed on the convex portions 66 with photosensitive polyimide.The size of a region (i.e., a pixel region) defined by the convexportions 66 was prescribed to be 100 μm×100 μm. Polyimide (JALS-204;produced by Japan Synthetic Rubber Co., Ltd.) was spin-coated onto theresultant substrate to form a homeotropic alignment layer 68.Furthermore, a homeotropic alignment layer (not shown) was also formedwith the same material on transparent electrodes of the other substrate.These substrates were attached to each other to complete a liquidcrystal cell.

In the present example, a mixture containing an n-type liquid crystalmaterial (Δε=−4.0; Δn=0.08; a chiral angle=90° in a cell gap of 5 μm),about 0.3 wt % of a compound A (photocurable resin) represented by thefollowing Formula I, and about 0.1 wt % of a polymerization initiator(Irgacure 651) was injected into the cell. Thereafter, a voltage of 5volts was applied to the cell to form axis-symmetrical alignment. Anaxis-symmetrical alignment region was formed in each pixel regiondefined by the convex portions 66, and a central axis was formed at acentral portion of each pixel region. Then, the cell was irradiated withUV-rays (intensity at 365 nm: about 6 mW/cm²) for 10 minutes at roomtemperature (25° C.) under the application of a voltage about 0.5 voltshigher than a threshold voltage of about 2.0 volts, whereby thephotocurable resin in the mixture was cured. As a result, theaxis-symmetrical alignment fixing layer 90 a was formed so as to coverthe homeotropic alignment layer 68. The axis-symmetrical alignment layer90 b (see FIG. 17) was also formed on the counter substrate.

The axis-symmetrical alignment of the cell in Example 8 did not returnto a homeotropic alignment state of the liquid crystal molecules evenwhen a voltage applied to the liquid crystal layer became less than ½V_(th). It is considered that the pretilt state in axis-symmetricalalignment was kept by the axis-symmetrical alignment fixing layers 90 aand 90 b. Thus, after the formation of the axis-symmetrical alignmentfixing layers 90 a and 90 b, a phenomenon that a plurality of centralaxes are present in each pixel region did not occur even when a voltageof ½ V_(th) or more was applied after removal of the voltage applied tothe liquid crystal layer, and the homeotropic alignment state (blackstate) and the axis-symmetrical alignment state (white state) was ableto be electrically controlled in a reversible manner. The liquid crystalmolecules contained in the liquid crystal layer of the liquid crystaldisplay device in Example 8 were provided with a pretilt angle by theaxis-symmetrical alignment fixing layer 90 a when no voltage is beingapplied. However, the shift from the homeotropic alignment was small, sothat a black level when no voltage is being applied was substantiallyequal to that of the liquid crystal display device in Example 1. Theelectro-optic characteristics and viewing angle characteristics were thesame as shown in FIGS. 13 and 14. Although a photocurable resin was usedin the present example, a thermosetting resin can also be used.

By providing a phase difference plate having a “Frisbee-type” refractiveoval body in the same way as in Example 7, wide viewing anglecharacteristics can be obtained as shown in FIG. 16. The phasedifference plate particularly improves the viewing angle characteristicsin a direction at an angle of 45° from polarization axes of polarizingplates.

Examples 9 and 10 and Comparative Examples 6 and 7

Liquid crystal display devices in Examples 9 and 10 and ComparativeExamples 6 and 7 were produced in the same way as in Example 8, byinjecting the mixtures with varying content of the above-mentionedcompound A. As is apparent from the results of Comparative Example 6,when the content of the photocurable resin was less than about 0.1 wt %,the axis-symmetrical alignment was not be able to be fixed effectively.When the content was more than about 6 wt %, the homeotropic alignmentof the liquid crystal molecules was disturbed, and light leakage becamelarge when no voltage is being applied. Thus, it is understood that thecontent of the photocurable resin is preferably in the range of about0.1 wt % to about 6 wt %.

TABLE 3 Comparative Example Example 8 9 10 6 7 Content of 0.3 0.1 2 0.056 Compound A (wt %) Transmittance 0.06 0.04 0.1 0.03 3.2 when no voltageis being applied (%) Fixing of axis- Good Good Good Poor Goodsymmetrical alignment

Example 11

FIG. 7 is a cross-sectional view of a PALC 400 in the present example.The PALC 400 includes a counter substrate 120, a plasma substrate 110,and a liquid crystal layer 102 disposed therebetween. The liquid crystallayer 102 is sealed with a sealant 106. The plasma substrate 110includes a substrate 111, a dielectric sheet 116 opposing the substrate111, and a plurality of plasma chambers 113 defined by partition walls112 provided between the substrate 111 and the dielectric sheet 116. Theplasma chambers 113 oppose the liquid crystal layer 102 with thedielectric layer 116 disposed therebetween. Gas sealed in each plasmachamber 113 is ionized by applying a voltage across an anode 114 and acathode 115 formed on the surface of the substrate 111 on the plasmachamber 113 side, whereby plasma discharge occurs. A plurality ofchambers 113 extend in the shape of stripes in a direction vertical tothe drawing surface of FIG. 7 in such a manner as to be orthogonal totransparent electrodes 105 formed on the surface of the countersubstrate 120 on the liquid crystal layer 102 side. Intersections of theplasma chambers 113 and the transparent electrodes 105 define pixelregions.

Convex portions 132 in the shape of a lattice are formed on the countersubstrate 120 on the liquid crystal layer 102 side so as to correspondto the non-pixel regions. The convex portions 132 allowaxis-symmetrically aligned regions to be formed so as to correspond tothe pixel regions. Furthermore, homeotropic alignment layers 134 a and134 b are provided on the surfaces of the plasma substrate 110 and thecounter substrate 120 on the liquid crystal layer 102 side.

The PALC 400 was produced as follows.

A plurality of electrodes each including a pair of anode 114 and cathode115, and the partition walls 112 with a height of about 200 μm wereformed with glass paste so as to make a partition between the adjacentelectrodes. Next, a thin film glass substrate 116 with a thickness ofabout 50 μm was attached to the partition walls 112 with a photocurablesealant. Thereafter, argon gas was sealed into the plasma chambers 113.The entire surface of the thin film glass substrate 116 was spin-coatedwith JALS-204 (produced by Japan Synthetic Rubber Co., Ltd.) to form thehomeotropic alignment layer 134 a, whereby the plasma substrate 110 wasobtained.

Referring to FIGS. 19A and 19B, a method for producing the countersubstrate 120 will be described. The convex portions 132 with a heightof about 2.7 μm were formed with OMR83 (produced by Tokyo Ohka-sha) onregions other than the pixel regions on a glass substrate 101 havingtransparent electrodes 105 made of ITO (thickness: about 150 nm) in theshape of stripes. The size of each pixel region was prescribed to be 100μm×100 μm. Furthermore, spacers 135 with a height of about 6 μm wereformed with photosensitive polyimide. The resultant substrate wasspin-coated with JALS-204 (produced by Japan Synthetic Rubber Co., Ltd.)to form the homeotropic alignment layer 134 b, whereby the countersubstrate 120 was obtained. The plasma substrate 110 was attached to thecounter substrate 120 to produce a liquid crystal cell.

An n-type liquid crystal material (Δε=−4.0, Δn=0.077; a twist anglepeculiar to the liquid crystal material=90° in a cell gap of 6 μm) wasinjected into the cell. A voltage of about 7 volts was applied to thecell. After application of the voltage, a plurality of central axes werepresent in an initial state. When the voltage was further continued tobe applied, one axis-symmetrically aligned region (monodomain) wasformed in each pixel region.

Polarizing plates 161 were disposed in crossed-Nicols on both sides ofthe cell, whereby a liquid crystal display device was produced. Thecross-sectional structure of the liquid crystal layer in the liquidcrystal display device thus obtained was substantially the same as thatof the liquid crystal display device shown in FIGS. 4A and 4B, exceptthat the cross-section of the homeotropic alignment layer 134 b had theshape of a mortar as shown in FIG. 19A (polarizing plates are notshown). Since the homeotropic alignment layer 134 b has a cross-sectionin the shape of a mortar, a differential coefficient of a curve showingchanges in thickness with respect to the position (from a centralportion of a pixel to a peripheral portion thereof) is positive, and adifferential coefficient of a curve showing changes in thickness of theliquid crystal layer in the pixel region is negative.

The axis-symmetrical alignment of the cell in Example 11 was stableunder the application of a voltage of ½ V_(th) or more, and wasdisturbed when the voltage was decreased to less than ½ V_(th) to returnto an initial state. When a voltage was applied to the cell again, aninitial axis-symmetrical alignment with a plurality of central axes wasobtained. Thereafter, an axis-symmetrical alignment state in which onecentral axis was formed in each pixel region was obtained. Thisphenomenon was obtained even when the same experiment was conducted 20times. After forming the axis-symmetrical alignment state by theapplication of a voltage of ½ V_(th) or more, the cell in Example 11 wasmeasured for electro-optic characteristics in a voltage range (½ V_(th)or more) in which the axis-symmetrical alignment was stable.

FIG. 13 shows the electro-optic characteristics thus obtained. As isapparent from FIG. 13, the liquid crystal display device of the presentinvention had a satisfactory contrast ratio (CR=300:1, 5 volts) with alow transmittance when no voltage is being applied. The thresholdvoltage was about 2 volts. A high contrast ratio was obtained in a wideviewing angle range as shown in FIG. 9. In FIG. 9, ψ represents anazimuth angle (i.e., intra-display surface angle), θ represents aviewing angle (i.e., a tilt angle from a normal to the display surface),and the hatched portion presents a region with a contrast of 10:1 ormore.

Comparative Example 8

In Comparative Example 8, the homeotropic alignment layer 134 b wasdirectly formed on the transparent electrode 105 formed on the surfaceof the substrate 101 shown in FIG. 19A. Thereafter, the spacers 135 wereformed using photosensitive polyimide in the same way as in Example 11.In Comparative Example 11, the convex portions 132 as shown in FIG. 19Awere not formed. The resultant counter substrate 120 was attached to theplasma substrate 110 formed in the same way as in Example 11 to producea liquid crystal cell. The thickness of the liquid crystal layer in thepixel regions in the cell was constant.

When the same material as that in Example 11 was injected into the cell,the liquid crystal molecules were randomly aligned, and disclinationlines were formed in a random manner. The cell was observed under theapplication of a voltage, showing that a display was rough in grayscales.

Example 12

In the present example, a method for stabilizing axis-symmetricalalignment of liquid crystal molecules by curing a photocurable resinmixed in a liquid crystal material will be described. FIG. 20 is aschematic partial cross-sectional view of a liquid crystal displaydevice in Example 12.

The liquid crystal display device 500 includes a liquid crystal layer 80containing an n-type liquid crystal material (liquid crystal molecules)92 having a negative dielectric anisotropy Δε between a pair ofsubstrates 82 and 84. A plasma substrate is used as either the substrate82 or 84. Homeotropic alignment layers 88 a and 88 b are provided on thesurfaces of the substrates 82 and 84 on the liquid crystal layer 80side. Convex portions 86 are formed on the surface of at least one ofthe substrates 82 and 84 on the liquid crystal layer 80 side. Since adielectric sheet provided on the plasma substrate on the liquid crystallayer 80 side is thin, the convex portions are preferably formed on thecounter substrate (color filter substrate) in view of the lack ofstrength of the dielectric sheet.

The liquid crystal layer 80 has two different thicknesses because of theconvex portions 86. Consequently, as described above, liquid crystalregions exhibiting axis-symmetrical alignment under the application of avoltage are defined by the convex portions 86. In FIG. 20, electrodesprovided on the substrates 82 and 84 for applying a voltage to theliquid crystal layer 80 and plasma chambers are omitted. The liquidcrystal display device 500 has the same structure as that of the liquidcrystal display device 400 in Example 11, except that theaxis-symmetrical alignment fixing layers 90 a and 90 b are provided onthe homeotropic alignment layers 88 a and 88 b. The axis-symmetricalalignment fixing layers 90 a and 90 b allow the liquid crystal moleculesin the pixel regions to keep axis-symmetrical alignment even when novoltage is being applied. Therefore, even when a voltage of less than ½V_(th) is applied (or a voltage is not applied) for driving the liquidcrystal display device 500, the electro-optic characteristics as shownin FIG. 2 can be obtained with good reproducibility. Theaxis-symmetrical alignment fixing layers 90 a and 90 b keepingaxis-symmetrical alignment (pretilt) of the liquid crystal molecules areformed by curing a curable resin mixed in a liquid crystal materialunder the application of a voltage of ½ V_(th) or more to the liquidcrystal layer.

Hereinafter, a method for producing the liquid crystal display device500 will be described in detail. Referring to FIG. 21, convex portions132 with a height of about 2.7 μm were formed with a photoresist (OMR83;produced by Tokyo Ohka-sha) on regions other than pixel regions of asubstrate 101 having transparent electrodes 105 made of ITO (thickness:150 nm) on its surface. Then, spacers 135 with a height of about 6 μmwere formed on the convex portions 132 with photosensitive polyimide.The size of a region (i.e., a pixel region) defined by the convexportions 132 was prescribed to be 100 μm×100 μm. Polyimide (JALS-204;produced by Japan Synthetic Rubber Co., Ltd.) was spin-coated onto theresultant substrate to form a homeotropic alignment layer 134 b toobtain a counter substrate. Furthermore, a homeotropic alignment layer(not shown) was also formed with the same material on transparentelectrodes of the other substrate (plasma substrate). These substrateswere attached to each other to complete a liquid crystal cell. This cellwas substantially the same as that in Example 11.

In the present example, a mixture containing an n-type liquid crystalmaterial (Δε=−4.0; Δn=0.077; a chiral angle=90° in a cell gap of 6 μm)and 0.4 wt % of a compound A (light curable resin) represented by thefollowing Formula I, and 0.1 wt % of Irgacure 651 was injected into thecell. Thereafter, a voltage of about 5 volts was applied to the cell toform axis-symmetrical alignment. An axis-symmetrical alignment regionwas formed in each pixel region defined by the convex portions 132, anda central axis was formed at a central portion of each pixel region.Then, the cell was irradiated with UV-rays (intensity at 365 nm: 6mW/cm²) for 10 minutes at room temperature (25° C.) under theapplication of a voltage about 0.5 volts higher than a threshold voltageof about 2.0 volts, whereby the photocurable resin in the mixture wascured. As a result, an axis-symmetrical alignment fixing layer 142 a wasformed so as to cover the homeotropic alignment layer 134 b. Theaxis-symmetrical alignment layer corresponding to the axis-symmetricalalignment layer 90 b (not shown in FIG. 21) was also formed on theplasma substrate. In the present example, although a photocurable resinwas used, a thermosetting resin can also be used.

The axis-symmetrical alignment of the cell in Example 12 did not returnto a homeotropic alignment state of the liquid crystal molecules evenwhen a voltage applied to the liquid crystal layer became less than ½V_(th). It is considered that the pretilt state in axis-symmetricalalignment was kept by the axis-symmetrical alignment fixing layer 142 a.Thus, after the formation of the axis-symmetrical alignment fixing layer142 a, a phenomenon that a plurality of central axes are present in thepixel regions did not occur even when a voltage of ½ V_(th) or more wasapplied after the applied voltage was removed from the liquid crystallayer, and the homeotropic alignment state (black state) and theaxis-symmetrical alignment state (white state) were able to beelectrically controlled in a reversible manner. The liquid crystalmolecules contained in the liquid crystal layer of the liquid crystaldisplay device in Example 12 were provided with a pretilt angle by theaxis-symmetrical alignment fixing layer 142 a when no voltage is beingapplied. However, the shift from the homeotropic alignment was small, sothat a black level when no voltage is being applied was substantiallyequal to that of the liquid crystal display device in Example 11. Theelectro-optic characteristics and viewing angle characteristics were thesame as shown in FIGS. 13 and 9. Although a photocurable resin was usedin the present example, a thermosetting resin can also be used.

In the liquid crystal display device of the present invention, twopolarizing plates were attached in such a manner that polarization axeswere placed in a crosswise direction on the display surface. As shown inFIG. 9, axis-symmetrical wide viewing angle characteristics wereobtained. Since the direction of the plasma chambers is identical withthat of the polarization axes of the polarizing plates, less lightleaked.

By providing a phase difference plate (Δn·d=300 nm) having a negative“Frisbee-type” refractive oval body between the cell and the polarizingplate, the viewing angle characteristics in a direction at an angle of45° from the polarization axes of the polarizing plates can be furtherimproved. Table 4 shows the results.

TABLE 4 Phase difference Phase difference plate is not plate is providedprovided Transmittance at 7% 55% a viewing angle of 60° in a directionat an angle of 45° from a polarization axis

Comparative Example 9

A liquid crystal cell was produced in the same way as in Example 11,except that the convex portions in the shape of a lattice were notformed on the counter substrate. Horizontal alignment films were formedon the surfaces of the substrates on the liquid crystal layer side, andthe horizontal alignment films were subjected to a rubbing treatment,whereby a liquid crystal cell in a TN mode was produced. A liquidcrystal material was injected into the cell, and the cell was heated andgradually cooled to produce TN-PALC. Polarizing plates were attached tothe cell in such a manner that polarization axes were shifted by 45°from a crosswise direction on the display surface. The viewing anglecharacteristics of the liquid crystal display device thus obtained wereas shown in FIG. 8A. As is understood from this figure, the viewingangle was much narrower, compared with those in Examples 11 and 12.Furthermore, light leakage from the attachment surface was observed inthe shape of lines. Thus, a contrast was decreased.

Example 13

FIG. 22A is a schematic cross-sectional view of one pixel of a liquidcrystal display device in Example 13. FIG. 22B is a plan view thereof.FIG. 22A is a cross-sectional view taken along the A—A line in FIG. 22B.The structure of the liquid crystal display device will be describedtogether with the production process.

Transparent electrodes 61 (thickness: about 100 nm) made of ITO wereformed on a glass substrate 60, and JALS-204 (produced by JapanSynthetic Rubber Co., Ltd.) was spin-coated onto the transparentelectrodes 61, whereby a homeotropic alignment layer 67 was formed.

Transparent electrodes 63 (thickness: about 100 nm) made of ITO wereformed on a glass substrate 62. The central portion of each pixel regionin the transparent electrodes 63 was removed by photolithography andetching to form an axis-symmetrical alignment central axis formingportion 64. Furthermore, convex portions 66 with a height of about 3 μmwere formed with an acrylic negative resist on regions other than thepixel regions on the transparent electrode 63. Thereafter, spacers 65with a height of about 2 μm were formed using photosensitive polyimide.The size of each pixel region defined by the spacers 65 and the convexportions 66 was prescribed to be 190 μm×325 μm. JAS-204 (produced byJapan Synthetic Rubber Co., Ltd.) was spin-coated onto the resultantsubstrate to form a homeotropic alignment layer 68.

Both the substrates 60 and 62 were attached to each other, and an n-typeliquid crystal material (Δε=−4.0, Δn=0.08, a twist angle peculiar to theliquid crystal material=90° in a cell gap of 5 μm) to form a liquidcrystal layer 70, whereby a liquid crystal cell was completed.

As the convex portions 66 and the spacers 65, photosensitive acrylatetype, methacrylate type, polyimide type, and rubber type materials maybe used. As long as the convex portions 66 and the spacers 65 areprovided with strength against the pressure of about 400 g/Φ, anyphotosensitive material may be used.

In order to perform an axis-symmetrical alignment central axis formingprocess, the cell thus produced was supplied with an axis-symmetricalalignment central axis forming voltage of about 7 volts. After theapplication of a voltage, a plurality of central axes were formed in aninitial state. When an axis-symmetrical alignment central axis formingvoltage was continued to be applied, one central axis was formed in eachpixel region, whereby one axis-symmetrical region (monodomain) wasformed.

Each pixel was observed in a transmission mode using a polarizingmicroscope in crossed Nicols under the application of a driving voltageto the cell. Some time after the commencement of application of thevoltage, it was observed that a plurality of central axes formed in aninitial state immediately after the application of a voltage became one.At this time, in about 10% of the pixel regions of the liquid crystalcell, the central axes were formed shifted from the central portions ofthe pixel regions. By continuing to apply an axis-symmetrical alignmentcentral axis forming voltage, the liquid crystal molecules wereaxis-symmetrically aligned around the central axis in each pixel regionduring a white display as shown in FIG. 23, and the central axes wereobserved to be formed at positions corresponding to the axis-symmetricalalignment central axis forming portions 64 in substantially centralportions of the pixel regions.

Polarizing plates 161 were disposed in crossed Nicols on both sides ofthe cell, whereby a liquid crystal display device was produced.

FIG. 13 shows electro-optic characteristics of the liquid crystaldisplay device in Example 13. FIG. 24 shows viewing anglecharacteristics of a contrast. FIG. 13 corresponds to FIG. 2. In FIG.24, ψ represents an azimuth angle (i.e., intra-display surface angle), θrepresents a viewing angle (i.e., a tilt angle from a normal to thedisplay surface), and the hatched portion represents a region with acontrast of 10:1 or more.

Example 14

FIG. 25A is a schematic cross-sectional view of a liquid crystal displaydevice in Example 14. FIG. 25B is a plan view thereof. FIG. 25A is across-sectional view taken along the A—A line in FIG. 25B.

In Example 14, a homeotropic alignment layer 68 provided above asubstrate 62 was formed so as to have a cross-section in a pixel regionsatisfying the relationship as shown in FIGS. 4A and 4B. That is, thehomeotropic alignment layer 68 was formed in such a manner that adifferential coefficient of a curve representing the changes inthickness of the homeotropic alignment layer 68 with respect to theposition (from a central portion of a pixel to a peripheral portionthereof) became positive, and a differential coefficient of a curverepresenting the changes in thickness of a liquid crystal layer in thepixel region became negative. More specifically, the cross-section ofthe homeotropic alignment layer 68 in the pixel region had the shape ofa mortar, and an axis-symmetrical alignment central axis forming portion64 was provided in a pixel electrode 63 at the deepest position of thecross-section of the homeotropic alignment layer 68. A liquid crystalcell was produced in the same way as in Example 13.

In order to perform an axis-symmetrical alignment central axis formingprocess, the cell thus produced was supplied with an axis-symmetricalalignment central axis forming voltage of about 7 volts. After theapplication of a voltage, a plurality of central axes were formed in aninitial state. When an axis-symmetrical alignment central axis formingvoltage was continued to be applied, one central axis was formed in eachpixel region, whereby one axis-symmetrical region (monodomain) wasformed.

Each pixel was observed in a transmission mode using a polarizingmicroscope in crossed Nicols under the application of a driving voltageto the cell. Some time after the commencement of application of thevoltage, it was observed that a plurality of central axes formed in aninitial state immediately after the application of a voltage became one.Each central axis thus formed was provided in a substantially centralportion of the pixel region corresponding to the deepest portion of themortar-shaped cross-section. By continuing to apply an axis-symmetricalalignment central axis forming voltage, the liquid crystal moleculeswere axis-symmetrically aligned around the central axis in each pixelregion during a white display as shown in FIG. 23, and the central axeswere observed to be formed at positions corresponding to theaxis-symmetrical alignment central axis forming portions 64 insubstantially central portions of the pixel regions.

Polarizing plates were disposed in crossed Nicols on both sides of thecell, whereby a liquid crystal display device was produced.

The liquid crystal display device in Example 14 had almost the sameelectro-optic characteristics and viewing angle characteristics of acontrast as those in Example 13.

Example 15

Liquid crystal cells were produced in the same way as in Example 13,except that the size of each pixel was prescribed to be 100 μm×100 μm,and the area of the axis-symmetrical alignment central axis formingportion 64 at a central portion of the pixel region was prescribed to be0 μm², 25 μm², 100 μm², 400 μm², and 900 μm². Polarizing plates wereplaced in crossed-Nicols on both sides of each cell, whereby liquidcrystal display devices were completed.

In the present example, each pixel was observed in a transmission modewith a polarizing microscope in crossed-Nicols under the application ofa driving voltage to the cells. The following Table 5 shows the resultsobtained by evaluating roughness of a display with each cell tiltedunder the application of a voltage which provides gray scales. In Table5, ◯ represents a display of good quality having almost no roughness; Δrepresents a display with negligible roughness; X represents a displaywith roughness; Sb represents an area of an axis-symmetrical alignmentcentral axis forming portion; and A represents an area of a pixelregion.

TABLE 5 Sb (μm²) A (μm²) Sb/A (%) Evaluation  0 10000 0 X  25 10000 0.25◯ 100 10000 1.0 ◯ 400 10000 4.0 Δ 900 10000 9.0 X

As is understood from Table 5, it is preferable that theaxis-symmetrical alignment central axis forming portion is provided sothat Sb satisfies 0<Sb/A<4%.

Example 16

A method for stabilizing the axis-symmetrical alignment state of liquidcrystal molecules by forming an axis-symmetrical alignment fixing layeron a surface of either one of substrates on a liquid crystal layer sidewill be described, the method including an axis-symmetrical alignmentcentral axis forming process in the course of the production of a liquidcrystal display device.

FIG. 26 is a schematic cross-sectional view of a liquid crystal displaydevice in Example 16. The liquid crystal display device in Example 16has the same structure as that in Example 13, except thataxis-symmetrical alignment fixing layers 90 a and 90 b are provided onhomeotropic alignment layers 68 and 67, respectively.

Substrates having cross-sectional structures shown in FIG. 26 wereproduced in the same way as in Example 13. Transparent electrodes 61(thickness: about 100 nm) made of ITO were formed on a glass substrate60. JALS-204 (produced by Japan Synthetic Rubber Co., Ltd.) wasspin-coated to form a homeotropic alignment layer 67.

Transparent electrodes 63 (thickness: about 100 nm) made of ITO wereformed on a glass substrate 62, and a central portion of a pixel regionwas removed by photolithography and etching, whereby an axis-symmetricalalignment central axis forming portion 64 was formed. Furthermore,convex portions 66 with a height of about 3 μm were formed with anacrylic negative resist outside of the pixel region on the transparentelectrode 63. Thereafter, spacers 65 with a height of about 2 μm wereformed with photosensitive polyimide. The size of the pixel regiondefined by the spacers 65 and the convex portions 66 was prescribed tobe 100 μm×100 μm. JALS-204 (produced by Japan Synthetic Rubber Co.,Ltd.) was spin-coated onto the resultant substrate, whereby ahomeotropic alignment layer 68 was formed.

The two substrates were attached to each other to complete a liquidcrystal cell. The structure of the cell thus obtained was the same asthat of the liquid crystal display device in Example 13.

In the present example, the following precursor mixture was injectedinto the cell thus produced. The precursor mixture contains an n-typeliquid crystal material (Δε=−4.0; Δn=0.08; a chiral angle=90° in a cellgap of 5 μm), and 0.3 wt % of compound A (photocurable resin)represented by the following Formula I, and 0.1 wt % of Irgacure 651.After the injection, an axis-symmetrical alignment central axis formingprocess was performed by applying an axis-symmetrical alignment centralaxis forming voltage of about 5 volts to the cell. Furthermore, the cellwas irradiated with UV-rays (intensity at 365 nm: 6 mW/cm²) at roomtemperature (25° C.) for 10 minutes under the application of theaxis-symmetrical alignment central axis forming voltage, whereby thephotocurable material in the precursor mixture was cured. As a result,the axis-symmetrical alignment fixing layers 90 a and 90 b were formedso as to cover the homeotropic alignment layers 68 and 67 on thesubstrates in the course of the axis-symmetrical alignment central axisforming process. The axis-symmetrical alignment fixing layers 90 a and90 b contain a polymer of a cured photocurable or thermosettingmaterial, such as an acrylate type material, a methacrylate typematerial, a styrene type material, and derivatives thereof, contained inthe precursor mixture.

Polarizing plates were attached to both sides of the cell to complete aliquid crystal display device.

Each pixel was observed in a transmission mode with a polarizingmicroscope in crossed-Nicols under the application of a voltage to thecell in Example 16. Even immediately after the application of a voltage,a single central axis was formed in each pixel region without aplurality of central axes being formed. Thereafter, the voltage appliedto the cell was once removed, and a voltage of ½ V_(th) or more wasapplied to the cell again. However, a phenomenon that a plurality ofcentral axes are present in each pixel region did not occur, and asingle central axis was formed. The reason for this is considered to beas follows: even when a voltage applied to the liquid crystal layerdecreased to less than ½ V_(th), the liquid crystal molecules did notreturn to a homeotropic alignment state; a pretilt state inaxis-symmetrical alignment was kept by the axis-symmetrical alignmentfixing layer 90 a. Thus, in the present example, a black display is ableto be performed when no voltage is being applied. Furthermore, it is notrequired to perform an axis-symmetrical alignment central axis formingprocess before a display operation. Although the liquid crystalmolecules were provided with a pretilt angle by the axis-symmetricalalignment fixing layer 90 a, the shift from homeotropic alignment wassmall. A black level when no voltage is being applied was substantiallythe same as that of the liquid crystal display device in Example 13. Theelectro-optic characteristics and viewing angle characteristics were thesame as those shown in FIGS. 13 and 24. In the present example, althougha photocurable resin was used, a thermosetting resin may be used.

Example 17

Liquid crystal display devices were produced in the same way as inExample 15 by injecting the precursor mixture with the varying contentof the compound A into the cell in Example 16.

The content of the compound A was varied from 0.05 wt % to 6 wt %. Thelight transmittance of the liquid crystal display devices when novoltage is being applied were measured, and the devices were observed tosee if a stable axis-symmetrical alignment state was formed.

As a result, when the content of the photocurable material was less thanabout 0.1 wt %, the axis-symmetrical alignment fixing process was notable to be performed effectively. When the content of the photocurablematerial was more than about 6 wt %, the homeotropic alignment of theliquid crystal molecules was disturbed to increase light leakage when novoltage is being applied. Thus, the content of the photocurable materialis preferably in the range of about 0.1 wt % to 6 wt %.

Example 18

In the present example, the following phase difference plates wereplaced between a pair of polarizing plates and a liquid crystal cell ofthe liquid crystal display device in Example 13 in such a manner that adelay axis of each phase difference plate was orthogonal to anabsorption axis of each polarizing plate.

The phase difference plate has optically negative birefringence, andsatisfies n_(x)=n_(y), n_(x)>n_(z), n_(y)>n_(z), where n_(x), n_(y) areprimary refractive indexes in an in-plane direction of a refractive ovalbody, and n_(z) is a primary refractive index in a thickness directionthereof.

Supposing that the thickness of the phase difference plate is d_(f), theretardation in the thickness direction was (n_(x)−n_(z))d_(f)=160 nm.

FIG. 27 shows the results obtained by measuring viewing anglecharacteristics of the liquid crystal display device in Example 18. InFIG. 27, ψ represents an azimuth angle (i.e., intra-display surfaceangle), θ represents a viewing angle (i.e., a tilt angle from a normalto the display surface), and the hatched portion represents a regionwith a contrast of 10:1 or more.

As is apparent from FIG. 27, the viewing angle of the liquid crystaldisplay device in the present example was larger than that of the liquidcrystal display device in Example 13 shown in FIG. 24, and the displayquality was uniform.

Example 19

In the present example, the following phase difference films were placedbetween a pair of polarizing plates and a liquid crystal cell of theliquid crystal display device in Example 13 in such a manner that adelay axis of each phase difference film was orthogonal to an absorptionaxis of each polarizing plate.

The phase difference film has optically negative birefringence, andsatisfies n_(x)>n_(y)>n_(z), where n_(x), n_(y) are primary refractiveindexes in an in-plane direction of a refractive oval body, and n_(z) isa primary refractive index in a thickness direction thereof.

Supposing that the thickness of the phase difference film is d_(f), theretardation in the thickness direction was (n_(z)−n_(y))d_(f)=170 nm.The retardation in an in-plane direction was (n_(x)−n_(y))d_(f)=42 nm.

FIG. 28 shows the results obtained by measuring viewing anglecharacteristics of the liquid crystal display device in Example 19. InFIG. 28, ψ represents an azimuth angle (i.e., intra-display surfaceangle), θ represents a viewing angle (i.e., a tilt angle from a normalto the display surface), and the hatched portion represents a regionwith a contrast of 10:1 or more.

As is apparent from FIG. 28, the viewing angle of the liquid crystaldisplay device in the present example was larger than that of the liquidcrystal display device in Example 13 shown in FIG. 24, and displayquality was uniform.

Comparative Example 10

In Comparative Example 10, as shown in FIG. 29, the homeotropicalignment layer 68 was directly formed on the transparent electrode 63provided on the surface of the substrate 62, and the spacers 65 wereformed with photosensitive polyimide in the same way as in Example 13.More specifically, in Comparative Example 10, the convex portions 66were not formed. The axis-symmetrical alignment central axis formingportion 64 was not formed, either.

The substrate on a lower side thus obtained was attached to a countersubstrate on an upper side formed in the same way as in Example 13 toproduce a liquid crystal cell. The same material as that in Example 13was injected into the cell, and polarizing plates were placed incrossed-Nicols on both sides of the cell.

In the liquid crystal display device in Comparative Example 10, theliquid crystal molecules were randomly aligned, and disclination lineswere randomly formed. The liquid crystal display device was observedunder the application of a voltage, a display with roughness wasobserved in gray scales.

Comparative Example 11

In Comparative Example 11, the homeotropic alignment layer 68 wasdirectly formed on the transparent electrode 63 provided on the surfaceof the substrate 62 shown in FIG. 22A. Thereafter, the spacers 65 wereformed with photosensitive polyimide in the same way as in Example 13.More specifically, in Comparative Example 11, the convex portions 66 asshown in FIG. 22A were not formed, and the axis-symmetrical alignmentcentral axis forming portion 64 was formed on the pixel electrode 63.

The substrate on a lower side thus obtained was attached to a countersubstrate on an upper side formed in the same way as in Example 13 toproduce a liquid crystal cell. The same material as that in Example 13was injected into the cell, and polarizing plates were placed incrossed-Nicols on both sides of the cell.

In the liquid crystal display device in Comparative Example 11, theliquid crystal molecules were randomly aligned, and disclination lineswere randomly formed in the same way as in Comparative Example 10. Theliquid crystal cell was observed under the application of anaxis-symmetrical alignment central axis forming voltage, a display withroughness was observed in gray scales.

As described above, according to the present invention, a liquid crystaldisplay device (including a plasma address LCD) with outstanding viewingangle characteristics and a high contrast, and a method for producingthe same are provided. The device includes a liquid crystal region whereliquid crystal molecules are vertically aligned when no voltage is beingapplied and are axis-symmetrically aligned under the application of avoltage.

The liquid crystal display device of the present invention hasoutstanding viewing angle characteristics because of a liquid crystalregion which is switched between homeotropic alignment andaxis-symmetrical alignment. Furthermore, the device uses a liquidcrystal material with a negative dielectric anisotropy, performing adisplay in a normally black mode in which a homeotropic alignment stateis obtained when no voltage is being applied. Therefore, a display witha high contrast can be provided. In particular, by controlling thepositions of the axis-symmetrical alignment central axes of the liquidcrystal molecules under the application of a voltage, roughness of adisplay in gray scales is eliminated, whereby display quality can beremarkably improved.

More specifically, since the convex portions defining the pixel regionsare formed on the surface of at least one of the substrates on theliquid crystal layer side, each pixel region exhibiting axis-symmetricalalignment is defined by the convex portions. Furthermore, a treatmentfor controlling the positions of the axis-symmetrical alignment centralaxes is performed, so that the position of the axis-symmetricalalignment central axis in each pixel region exhibiting axis-symmetricalalignment is defined.

Examples of the treatment for controlling the positions of theaxis-symmetrical alignment central axes include: (i) performing anaxis-symmetrical alignment central axis forming process in which adesired voltage is applied for a desired period of time or longer; (ii)prescribing Sa so as to satisfy 0<Sa/A<4%, where Sa represents an areaof a region in which the liquid crystal molecules keep a homeotropicalignment state under the application of an axis-symmetrical alignmentcentral axis forming voltage and A represents an area of a pixel region;(iii) forming an axis-symmetrical alignment central axis forming portionat a substantially central position or at a predetermined position ineach of a plurality of pixel regions; (iv) prescribing the thickness ofthe liquid crystal layer in the pixel region so as to become largest ata central portion of the pixel region and continuously decrease from thecentral portion to a peripheral portion of the pixel region; and (v)forming an axis-symmetrical alignment fixing layer on the surface of atleast one of the substrates on the liquid crystal layer side.

The liquid crystal display device of the present invention is preferablyused, for example, for a portable information terminal used by a numberof people, a personal computer, a word processor, amusement equipment,education equipment, a flat display used in a TV apparatus, and adisplay plate, window, door and wall utilizing a shutter effect. Theliquid crystal display device of the present invention is alsopreferably used as a large display apparatus such as a high definitionTV (HDTV) and a display for a CAD.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A plasma addressed liquid crystal display devicehaving a plasma address (PALC) construction, comprising: a countersubstrate including a first substrate having signal electrodes, adielectric sheet opposing the first substrate, and a liquid crystallayer provided between the dielectric sheet and the first substrate; aplasma substrate comprising a second substrate, the dielectric sheetopposing the second substrate, and plasma chambers disposed between thesecond substrate and the dielectric sheet for performing plasmadischarge; a liquid crystal layer disposed between the counter substrateand the plasma substrate; the device being driven by the signalelectrodes and plasma chambers, wherein liquid crystal molecules in theliquid crystal layer have a negative dielectric anisotropy, and theliquid crystal molecules are aligned in a direction substantiallyvertical to the substrates when no voltage is being applied, and theliquid crystal molecules are axis-symmetrically aligned in each of aplurality of pixel regions under application of a voltage, whereinhomeotropic alignment layers are respectively provided on a surface ofthe first substrate and dielectric sheet side of the liquid crystallayer, and said homeotropic alignment layers axially-symmetrical alignthe liquid crystal molecules about a central axis of each pixel regionupon application of said voltage, and wherein a thickness of the liquidcrystal layer in the pixel region is largest in a central portion of thepixel region and continuously decreases toward a peripheral portion ofthe pixel region.
 2. A plasma addressed liquid crystal display deviceaccording to claim 1, wherein the homeotropic alignment layer on thesurface of the plasma substrate is in a region corresponding to theplurality of pixel regions on a surface of at least one of thesubstrates on the liquid crystal layer side.
 3. A liquid crystal displaydevice according to claim 2, wherein at least one of the countersubstrate and the plasma substrate has convex portions defining theplurality of pixel regions on a surface on the liquid crystal layerside.
 4. A liquid crystal display device according to claim 1,comprising a pair of polarizing plates disposed in crossed-Nicolsalignment on opposite sides of the liquid crystal layer, a polarizationaxis of one of the polarizing plates being parallel to an extendingdirection of the signal electrodes of the plasma chambers.
 5. A liquidcrystal display device according to claim 1, wherein an axis-symmetricalalignment fixing layer which provides the liquid crystal molecules withan axis-symmetrical pretilt angle is on a liquid crystal layer side ofat least one of the plasma substrate and the counter substrate.
 6. Aliquid crystal display device according to claim 5, wherein theaxis-symmetrical alignment fixing layer contains a photocurable resin.7. A liquid crystal display device comprising: plasma substratecomprising a substrate, a dielectric sheet opposing the substrate, andplasma chambers for performing plasma discharge between the substrateand dielectric sheet; a counter substrate having signal electrodes; aliquid crystal layer provided between the dielectric sheet and thecounter substrate, the device being driven by the signal electrodes andthe plasma chambers, wherein liquid crystal molecules in the liquidcrystal layer have a negative dielectric anisotropy, and the liquidcrystal molecules are aligned in a direction substantially vertical tothe substrates when no voltage is being applied, and the liquid crystalmolecules are axis-symmetrically aligned in each of a plurality of pixelregions under application of a voltage, and wherein a thickness of theliquid crystal layer in the pixel region is largest at a central portionof the pixel region and continuously decreases toward a peripheralportion in each of the plurality pixel regions.
 8. A liquid crystaldisplay device according to claim 7, wherein the thickness of the liquidcrystal layer in the pixel region is axis-symmetrically aligned aroundthe central portion of the pixel region.